Aluminum alloy plate having excellent moldability and bake finish hardening properties

ABSTRACT

An aluminum alloy sheet excellent in terms of formability and bake hardenability is provided. The aluminum alloy sheet contains, in terms of mass %, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0% and Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable impurities. A differential scanning calorimetry curve of the aluminum alloy sheet has an endothermic peak in a temperature range of 150 to 230° C. and an exothermic peak in a temperature range of 240 to 255° C. The endothermic peak corresponds to a dissolution of a Mg—Si cluster and has a peak height of 8 μW/mg or less, including 0 μW/mg. The exothermic peak corresponds to a formation of a Mg—Si cluster and has a peak height of 20 μW/mg or larger.

TECHNICAL FIELD

The present invention relates to an Al—Mg—Si alloy sheet. The aluminum alloy sheet referred to in the present invention means an aluminum alloy sheet that is a rolled sheet such as a hot rolled sheet or a cold rolled sheet and has been subjected to refining such as a solution heat treatment and a quenching treatment, but is not yet subjected to a press forming and a bake hardening treatment. Further, aluminum is hereinafter also referred to as Al.

BACKGROUND ART

In recent years, because of environmental awareness and the like, the society's requirement for weight reduction in a vehicle such as an automobile has been steadily increasing. In order to respond to such requirement, as a material for a large body panel structure (an outer panel or an inner panel) of an automobile instead of a steel material such as a steel sheet, application of an aluminum alloy material excellent in formability and bake hardenability and lighter in weight has been increasing.

Among the large body panel structure of an automobile, for an outer panel (outer sheet) such as a hood, a fender, a door, a roof, or a trunk lid, use of an Al—Mg—Si-based AA or HS 6000-series (hereinafter, also simply referred to as a 6000-series) aluminum alloy sheet, as a thin and high strength aluminum alloy sheet, has been studied.

The 6000-series aluminum alloy sheet contains Si and Mg as essential components. In particular, a 6000-series aluminum alloy with excess Si has a composition in which the Si/Mg mass ratio is 1 or greater, and has excellent age hardenability. Because of this, formability for press forming or bending into the outer panels of automobiles is secured by lowering the proof stress. In addition, it has such bake hardenability (hereinafter referred to also as BH response) that it undergoes age hardening upon heating in an artificial aging (hardening) treatment performed at a relatively low temperature, such as the baking treatment of formed panels, and hence improves in proof stress, thereby ensuring the strength required as a panel.

On the other hand, as is known well, an outer panel of an automobile is manufactured by applying combined formings, such as stretch forming or bending forming in press forming, to an aluminum alloy sheet. For example, in a large outer panel such as a hood or a door, the shape of a formed product is made as an outer panel by press forming such as stretching, and then joining with an inner panel is executed by hem work (hemming) of a flat hem and the like of the outer panel peripheral section to be formed into a panel structural body.

Here, the 6000-series aluminum alloy had an advantage of having excellent BH response, but had a problem of having aging properties at room temperature, that is, of age hardening during retention at room temperature after solution heat treatment and quenching treatment to increase the strength, thereby deteriorating formability into a panel, particularly the bendability. For example, in a case where a 6000-series aluminum alloy sheet is to be used for an automobile panel, it is placed at room temperature (standing at room temperature) for approximately 1 month after the solution heat treatment and the quenching treatment (after manufacturing) at an aluminum manufacturer until forming into a panel at an automobile manufacturer, and comes to be significantly age hardened (room-temperature aged) during that time. Particularly, in the outer panel subjected to severe bending, there was such a problem that, although forming was possible without any problem immediately after manufacturing, cracking occurred in hem working after the lapse of 1 month. Therefore, in the 6000-series aluminum alloy sheet for an automobile panel, particularly for an outer panel, it is necessary to suppress room-temperature aging over a comparatively long period of approximately 1 month.

Moreover, in the case where such room-temperature aging is great, a problem also occurs in that the BH response deteriorate and the proof stress is not improved to the strength required as a panel by heating during an artificial aging (hardening) treatment at a comparatively low temperature, such as a bake treatment and the like of the panel after forming described above.

Hereto, from the standpoint of the structure of 6000-series aluminum alloy sheets, in particular, the compounds (crystals or precipitates) formed by elements contained therein, various proposals have been made on property improvements such as improvements in formability or BH response and inhibition of room-temperature aging. Recently, in particular, it has been proposed to make an attempt to directly examine and control clusters (aggregates of atoms) which affect the BH response and room-temperature aging properties of 6000-series aluminum alloy sheets (Patent Documents 1 to 3).

Among these, in Patent Document 1, clusters (aggregates of atoms) which affect BH response and room-temperature aging properties are analyzed through a direct examination of the structure of the sheet as such with a transmission electron microscope at a magnification of one million and, among the clusters (aggregates of atoms) observed, the average number density of clusters having a circle equivalent diameter of in the range of 1 to 5 nm is regulated so as to be in a certain range, thereby attaining excellent BH response and suppressed room-temperature aging.

In contrast, in Patent Documents 2 and 3, in place of directly examining clusters (aggregates of atoms) as in Patent Document 1, by using a three-dimensional atom probe field ion microscope and from positional information of atoms of the sheet, temporarily ionized in a high electric field (electric-field evaporation), aggregates of atoms are specified, which are defined in relation to a structure of atoms in the sheet reconstructed by the analysis. More specifically, atom aggregates are controlled so that they include ten or more pieces of either or both of Mg atom and Si atom in total and satisfies a requirement that when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less. Namely, the average number density, size distribution or proportion of atom aggregates which satisfy these requirements is specified.

Furthermore, in prior patent documents which are relevant to the addition of Sn according to the present invention, many methods have been proposed in which Sn is positively added to a 6000-series aluminum alloy sheet to suppress room-temperature aging and improve BH response (bake hardenability). For example, Patent Document 4 proposes a method in which Sn is added in an appropriate amount and a solution treatment and subsequently preliminary aging are performed to thereby obtain both of suppressed room-temperature aging properties and BH response. Patent Document 5 proposes a method in which Sn and Cu, which improves formability, are added to improve formability, BH response and corrosion resistance.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-2009-242904 -   Patent Document 2: JP-A-2012-193399 -   Patent Document 3: JP-A-2013-60627 -   Patent Document 4: JP-A-09-249950 -   Patent Document 5: JP-A-10-226894

SUMMARY OF THE INVENTION Problem that the Invention is to Solve

However, even in those conventional Al—Mg—Si alloy sheets, there has still been room for obtaining both of satisfactory formability and high BH response after long-term room-temperature aging.

The various outer panels for automobiles are required, from the standpoint of design, to attain strain-free, beautiful curved-surface configurations and character lines. Such requirements are becoming severer year by year since high-strength aluminum alloy sheet materials which are difficult to form are being adopted for the purpose of weight reduction. There is hence a growing desire in recent years for an aluminum alloy sheet having even better formability. However, with the conventional structure control described above, it is impossible to meet such requirements.

For example, one cause which renders high-strength aluminum alloy sheets difficult to apply to such outer panels is the problem concerning the shapes peculiar to outer panels. Recessed portions having given depths (protrudent portions, embossed portions) for attaching devices or members, such as knob mount bases, lamp mount bases and license (number plate) mount bases, or for drawing wheel arches are partly provided to outer panels.

In the cases when such a recessed portion is press-formed together with consecutive curved surfaces around the recessed portion shape, face strains are prone to occur and it is difficult to attain the strain-free, beautiful curved-surface configuration and character line. Consequently, it is essential for the outer panels that the occurrence of such face strains should be inhibited when the raw sheets are formed.

The problem of such face strains is not a problem only for those recessed portions (protrudent portions) but a problem common to automotive panels which partly have a recessed portion (protrudent portion) that may suffer a face strain, such as a saddle-shaped portion of a door outer panel, a vertical wall portion of a front fender, a window corner portion of a rear fender, a character-line termination portion of a trunk lid or hood outer panel, and a root portion of a rear fender pillar.

From the standpoint of attaining improved formability for inhibiting the occurrence of the face strains to overcome the conventional problem described above, it is desirable that a sheet in press forming (having undergone room-temperature aging after production) should have a 0.2% proof stress reduced to less than 110 MPa. However, in the cases when the proof stress in forming has been reduced as the above, it is difficult to attain a 0.2% proof stress of 200 MPa or greater after bake hardening (hereinafter also referred to as “after BH”), that is, to attain an increase in 0.2% proof stress through bake hardening of 100 MPa or greater. With the conventional structure control with a DSC described above, it is difficult to overcome the problem.

A first aspect of the present invention has been achieved in order to overcome the conventional problem. An object thereof (hereinafter referred to also as first object) is to provide an aluminum alloy sheet which combines formability and bake hardenability, that is, which can have, in automotive-panel forming, a 0.2% proof stress reduced to 110 MPa or less and can have a 0.2% proof stress after BH of 200 MPa or greater.

Meanwhile, from the standpoint of attaining improved formability for inhibiting the occurrence of the face strains to overcome the conventional problem, it is desirable that a sheet in press forming (having undergone room-temperature aging after production) should have, not only a 0.2% proof stress reduced to 110 MPa or less but also a reduced value of yield ratio, which is the ratio between tensile strength and yield strength [(0.2% proof stress)/(tensile strength)]. However, in the cases when the proof stress in forming has been reduced as the above, it is difficult to attain a 0.2% proof stress of 190 MPa or greater after bake hardening treatment (hereinafter referred to also as “BH”), that is, to attain an increase in 0.2% proof stress through bake hardening of 100 MPa or greater.

A second aspect of the present invention has been achieved in order to overcome the conventional problem. An object thereof (hereinafter referred to also as second object) is to provide an aluminum alloy sheet which can not only have, in automotive-panel forming, a 0.2% proof stress reduced to 110 MPa or less and a yield ratio reduced to less than 0.50 but also have a 0.2% proof stress after BH of 190 MPa or greater to thereby combines formability and bake hardenability and attains both an increase in BH response and a reduction in yield ratio.

Means for Solving the Problem

The gist of the aluminum alloy sheet according to the first aspect of the present invention, which is for achieving the first object and is excellent in terms of formability and bake hardenability, is an Al—Mg—Si alloy sheet containing, in terms of mass %, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0% and Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable impurities, in which a differential scanning calorimetry curve of the aluminum alloy sheet has an endothermic peak in a temperature range of 150 to 230° C., that is an endothermic peak corresponding to a dissolution of a Mg—Si cluster and that has a peak height of 8 μW/mg or less (including 0 μW/mg), and has an exothermic peak in a temperature range of 240 to 255° C., that is an exothermic peak corresponding to a formation of a Mg—Si cluster and that has a peak height of 20 μW/mg or larger.

The differential thermal analysis at each of measurement portions in the aluminum alloy sheet is performed under the same conditions including a test apparatus of DSC220G, manufactured by Seiko Instruments Inc., a reference substance of aluminum, a sample container made of aluminum, temperature increase conditions of 15° C./min, an atmosphere of argon (50 mL/min), and a sample weight of 24.5 to 26.5 mg. The differential thermal analysis profile (μW) obtained is divided by the sample weight and thereby normalized (μW/mg). Thereafter, in the range of 0 to 100° C. in the differential thermal analysis profile, a region where the differential thermal analysis profile is horizontal is taken as a reference level of 0, and the height of exothermic peak from the reference level is measured.

The gist of the aluminum alloy sheet according to the second aspect of the present invention, which is for achieving the second object and is excellent in terms of formability and bake hardenability, is an Al—Mg—Si alloy sheet containing, in terms of mass %, Mg: 0.3 to 1.0%, Si: 0.5 to 1.5% and Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable impurities, in which a solid-solution amount of Mg+Si in a solution, separated by a residue extraction method with hot phenol is 1.0 mass % or more and 2.0 mass % or less, and

in which atom aggregates observed with a three-dimensional atom probe field ion microscope satisfy conditions that either or both of an Mg atom and an Si atom are contained therein by a total of 10 pieces or more and that, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, and regarding the atom aggregates, an average volume proportion (ΣVi/V_(Al))×100 is in a range of 0.3 to 1.5%, the average volume proportion (ΣVi/V_(Al)) being a proportion of the total volume of the atom aggregates, in terms of the total volume ΣVi obtained by summing up volumes of the individual atom aggregates Vi (=4/3πr_(G) ³) calculated from a Guinier radius r_(G) of the individual atom aggregates each regarded as a sphere, to a volume V_(Al) of the aluminum alloy sheet measured with the three-dimensional atom probe field ion microscope, in which

an average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 is 20 to 70%, the average volume proportion (ΣVi_(1.5 or more)/ΣVi) being a proportion of a total volume ΣVi_(1.5 or more) obtained by summing up volumes V_(1.5 or more) of atom aggregates each having the Guinier radius r_(G) of 1.5 nm or larger to a total volume of the atom aggregates ΣVi.

Effects of the Invention

With regard to the first aspect, Sn exerts such effects in the structure of the Al—Mg—Si alloy sheet that, at room temperature, it captures (traps) atomic holes and thereby inhibits diffusion of Mg and Si at room temperature, inhibits the strength from increasing at room temperature and, during the forming of the sheet into panels, improves the press formability including hem workability, drawability and punch stretch formability (hereinafter, this press formability is referred to also as hem workability as a representative). During an artificial aging treatment of the panels, such as a baking treatment, it releases the captured holes and hence in turn enhances the diffusion of Mg and Si. Consequently, the BH response can be enhanced.

However, the present inventors have found that the addition of such Sn poses a new problem due to peculiar properties of Sn. Specifically, in the cases when Sn is added and a sheet is produced by an ordinary method, the addition of Sn rather leads, depending on the production conditions, to a decrease in the amount of Mg—Si clusters which contribute to strength. There are hence cases where the addition of Sn results in an insufficient amount of precipitates which precipitate after a bake hardening treatment, making it impossible to obtain the strength required as automotive panels as described above.

The reason for this is presumed to be because the Sn's effect of capturing and releasing atomic holes is related with the fact that the solid-solution amount of Sn in the matrix is exceedingly small (in an ordinary means, even when the added amount of Sn is controlled to equal to or less than a theoretical solid-solution amount, a large proportion thereof crystallizes out or precipitates as compounds without coming into a solid-solution state). However, this presumption is uncertain.

In any case, the addition of Sn itself may become meaningless unless problems such as the decrease in the amount of Mg—Si clusters which contribute to strength and the insufficient amount of precipitates which precipitate after a bake hardening treatment are overcome, such problems being regarded as side effects of the addition of Sn.

Because of this, in the present aspect, the inventors have ventured to reconsider sheet production processes and contrived production conditions concerning, for example, a preliminary aging treatment (reheating treatment) after a solution quenching treatment as will be described later, so that addition of Sn does not result in a decrease in the amount of Mg—Si clusters which contribute to strength or in an insufficient amount of precipitates which precipitate after a bake hardening treatment.

The inventors have further discovered that a DSC (differential scanning calorimetry curve) of this sheet can be applied as a standard of the structure which can, even when Sn has been added thereto, prevent the Mg—Si clusters that contribute to strength from being diminished and increase or ensure the amount of precipitates that precipitate after a bake hardening treatment. Specifically, in the present aspect, based on the DSC, an endothermic peak corresponding to the dissolution of relatively small Mg—Si clusters, which do not contribute to strength, is controlled and meanwhile an exothermic peak corresponding to the formation of relatively large Mg—Si clusters, which contribute to strength, is enhanced. Thus, Mg—Si clusters that do not contribute to strength are suppressed and the Mg—Si clusters that contribute to strength are increased, thereby obtaining desired BH response.

As a result, according to the present aspect, it is possible to provide an aluminum alloy sheet which combines formability and bake hardenability and which contains Sn and can have a 0.2% proof stress in automotive-panel forming reduced to 110 MPa or less and have a 0.2% proof stress after BH of 200 MPa or greater.

In the second aspect, first in order to ensure formability of the Al—Mg—Si alloy sheet into the outer panels (hereinafter, this press formability is referred to also as hem workability as a representative), the sheet in forming is aimed to have a 0.2% proof stress reduced to 110 MPa or less and a yield ratio reduced to less than 0.50.

Because of this, the solid-solution amount of Mg and Si is controlled in the present aspect in addition to the alloy composition including Mg and Si. Furthermore, by adding Sn, the BH response is enhanced while ensuring the formability. As will be described later, Sn has an important effect of attaining both an increase in BH response and a reduction in yield ratio by reducing the volume proportion of atom aggregates which inhibit the yield ratio from being reduced, even when the solid-solution amount of Mg+Si is increased.

Furthermore, in the present aspect, the size distribution of atom aggregates observed with a three-dimensional atom probe field ion microscope is further specified in order to control Mg—Si atom aggregates, in addition to the means described above, so that the yield ratio during sheet forming can be reliably reduced to less than 0.50.

The term “atom aggregates observed with a three-dimensional atom probe field ion microscope” here means known atom aggregates including the measurement methods described in Patent Documents 2 and 3, and does not mean atom aggregates (clusters) observed by directly examining the sizes and state thereof in the structure of the sheet with a high-magnification TEM by using the structure of the sheet as such, as in Patent Document 1. In other words, as in Patent Documents 2 and 3, those are the atom aggregates in a three-dimensional structure of atoms (three-dimensional atom map) obtained by a reconstruction through analysis from the flight times and positions of atoms of the sheet which have temporarily ionized in a high electric field (electric-field evaporation) with a three-dimensional atom probe field ion microscope, as the details of the measuring method will be described later. Those are the atom aggregates which are defined to satisfy the given requirements specified in claim 1 (that is, deemed to be atom aggregates) in the three-dimensional structure of atoms.

In the present aspect, in order to reduce a yield ratio to less than 0.50, as a size distribution of the atom aggregates observed with a three-dimensional atom probe field ion microscope, the proportion of atom aggregates that satisfy the requirements that Mg atom and/or Si atom is contained and the distance between the atoms are 0.75 nm or less, is regulated so as to be in a certain range in terms of volume proportion. In addition, the proportion of relatively large atom aggregates which each have a Guinier radius r_(G) of 1.5 nm or larger, among those atom aggregates, is increased in a certain range in terms of volume proportion.

As a result, according to the present aspect, it is possible to provide an aluminum alloy sheet which combines formability and bake hardenability and which contains Sn and can not only have, in automotive-panel forming, a 0.2% proof stress reduced to 110 MPa or less and a yield ratio reduced to less than 0.50 but also have a 0.2% proof stress after BH of 190 MPa or greater.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an explanatory view which shows each DSC of Examples according to the first aspect.

MODES FOR CARRYING OUT THE INVENTION (First Aspect)

The first aspect of the present invention will be explained below in detail with respect to each requirement.

(Chemical Component Composition)

First, the chemical component composition of the Al—Mg—Si (hereinafter referred to also as 6000-series) aluminum alloy sheet according to the present aspect is explained below. The 6000-series aluminum alloy sheet targeted by the present aspect, as, for example, the sheet for the automotive outer panels, is required to have various properties such as excellent formability, BH response, strength, weldability, and corrosion resistance. Consequently, such requirements are also met by means of the composition. In addition, in the present aspect, Sn is incorporated to suppress the room-temperature aging of the sheet after production and to reduce a 0.2% proof stress in the panel forming to 110 MPa or less. Thus, the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures, can be improved. Simultaneously therewith, a 0.2% proof stress after bake hardening of 200 MPa or greater is rendered possible by means of the composition.

In order to satisfy such requirements, the aluminum alloy sheet according to the present aspect has a composition which includes, in terms of mass %, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0% and Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable impurities. All the content indicated in % of the elements means that in mass %. In this description, percentage on mass basis (mass %) is the same as percentage on weight basis (wt %). With respect to the content of a chemical component, there are cases where “X % or less (exclusive of 0%)” is expressed by “more than 0% and X % or less”.

In the present aspect, elements other than Mg, Si and Sn are impurities or elements which may be contained, and may have contents (permissible amounts) on levels of the elements in accordance with the AA or HS standards, etc.

Namely, there are cases, in the present aspect also, where not only high-purity Al base metal but also 6000-series alloys containing elements other than Mg and Si as additive elements (alloying elements) in large amounts, other aluminum alloy scrap materials, low-purity Al base metal, and the like are used in large quantities as melted raw materials for the alloy, from the standpoint of resource recycling. In such cases, other elements such as shown below are inevitably included in substantial amounts. Since refining performed for intentionally diminishing these elements itself leads to an increase in cost, it is necessary to accept some degree of inclusion. There are useful content ranges which permit inclusion of such elements in substantial amounts but do not inhibit the object or effects of the present aspect.

Consequently, in the present aspect, inclusion of such elements shown below is permissible within the range of equal to or less than the upper limits specified below, which are in accordance with the AA or JIS standards or the like.

Specifically, the aluminum alloy sheet may further contain one kind or two or more kinds selected from the group consisting of Fe: 1.0% or less (exclusive of 0%), Mn: 1.0% or less (exclusive of 0%), Cr: 0.3% or less (exclusive of 0%), Zr: 0.3% or less (exclusive of 0%), V: 0.3% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%), Cu: 1.0% or less (exclusive of 0%), Ag: 0.2% or less (exclusive of 0%), and Zn: 1.0% or less (exclusive of 0%), within those ranges, in addition to the basic composition shown above.

In the cases where these elements are contained, the content of Cu is preferably 0.7% or less and more preferably 0.3% or less, because Cu is prone to impair the corrosion resistance when contained in a large amount. Mn, Fe, Cr, Zr, and V are prone to yield relatively coarse compounds when contained in large amounts, and are prone to impair the hem workability (hem bendability), which is addressed by the present aspect. Consequently, the content of Mn is preferably 0.6% or less and more preferably 0.3% or less, and the content of each of Cr, Zr and V is preferably 0.2% or less and more preferably 0.1% or less.

The content range of each element and the purposes and permissible amount thereof in the 6000-series aluminum alloy are explained below in order.

Si: 0.3 to 2.0%

Si, together with Mg, is an essential element for obtaining the strength (proof stress) required as automotive panels by forming aging precipitates which contribute to an improvement in strength, during an artificial aging treatment such as a baking treatment, and thus exhibiting an age hardenability. In the case where the addition amount of Si is too small, the amount of precipitates after artificial aging is too small and the increase in strength through baking is too small. Meanwhile, in the case where the content of Si is too high, the Si forms coarse crystals with impurity Fe, etc., resulting in a considerable decrease in formability such as bendability. In addition, too high Si contents increases not only the strength just after sheet production but also the room-temperature aging amount after the production, thereby increases the strength before forming too much, and reduces the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures. Consequently, the content of Si is regulated so as to be in the range of 0.3 to 2.0%.

For attaining an excellent age hardenability in a baking treatment performed at a lower temperature for a shorter period after forming into panels, it is preferable to employ a 6000-series aluminum alloy composition in which Si/Mg is 1.0 or larger in terms of mass ratio so that Si has been incorporated further excessively relative to the Mg than in the so-called excess-Si type.

Mg: 0.2 to 2.0%

Mg, together with Si, is also an important element for forming the clusters specified in the present aspect. It is an essential element for obtaining the proof stress required as panels by forming, together with the Si, aging precipitates which contribute to an improvement in strength, during an artificial aging treatment such as a baking treatment, and thus exhibiting an age hardenability. In the case where the content of Mg is too low, the amount of precipitates after artificial aging is too small and the strength after baking is thus too low. Meanwhile, in the case where the content of Mg is too high, the Mg forms coarse crystals with impurity Fe, etc., resulting in a considerable decrease in formability such as bendability. In addition, too high Mg contents increases not only the strength just after sheet production but also the room-temperature aging amount after the production, thereby increases the strength before forming, and reduces the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures. Consequently, the content of Mg is regulated so as to be in the range of 0.2 to 2.0%.

Sn: 0.005 to 0.3%

Sn, at room temperature, has the effect of capturing (trapping) atomic holes to thereby inhibit room-temperature diffusion of Mg and Si and inhibit the strength from increasing at room temperature, and during the forming of the sheet into panels, improving the press formability including hem workability, drawability and punch stretch formability (hereinafter, this press formability is referred to also as hem workability as a representative). During an artificial aging treatment of the panels, such as a baking treatment, it releases the captured holes and hence in turn enhances the diffusion of Mg and Si, thereby enhancing the BH response. In the case where the content of Sn is lower than 0.005%, the Sn cannot sufficiently trap holes and is unable to exhibit the effects thereof. Meanwhile, in the case where the content of Sn is higher than 0.3%, the Sn segregates at grain boundaries and it is prone to cause intergranular cracks. A preferred lower limit of the content of Sn is 0.01%. An upper limit of the content of Sn is preferably 0.2%, more preferably 0.1% and further preferably 0.06%.

(Structure)

The composition described above is employed and furthermore, in the present aspect, the 6000-series aluminum alloy sheet is made to have the following structure. In order to ensure high strength as automotive panels or the like, a DSC of this sheet is used as a measure for ensuring the amount of precipitates which precipitate after a bake hardening treatment, and an endothermic peak and an exothermic peak in specific temperature ranges, which affect, in particular, the strength before baking and an increase in strength through the baking are controlled. In other words, a DSC of this sheet is used to control an endothermic peak and an exothermic peak in specific temperature ranges, which affect, in particular, the strength before baking and an increase in strength through the baking, so that the addition of Sn does not result in a decrease in the amount of Mg—Si clusters which contribute to strength or result in insufficient amount of precipitates which precipitate after a bake hardening treatment.

More specifically, in the present aspect, based on the DSC, an endothermic peak corresponding to the dissolution of relatively small Mg—Si clusters, which do not contribute to strength, is controlled and meanwhile an exothermic peak corresponding to the formation of relatively large Mg—Si clusters, which contribute to strength, is enhanced. Thus, Mg—Si clusters that do not contribute to strength are suppressed and the Mg—Si clusters that contribute to strength are increased, thereby obtaining desired BH response.

Here, the differential scanning calorimetry curve (DSC) is a heating curve from solid phase, obtained by measuring the thermal changes during melting of aluminum alloy sheet after the refining treatment, by differential thermal analysis performed under the following conditions.

Specifically, the differential thermal analysis at each of measurement portions in the aluminum alloy sheet is performed under the same conditions including a test apparatus of DSC220G, manufactured by Seiko Instruments Inc., a reference substance of aluminum, a sample container made of aluminum, temperature increase conditions of 15° C./min, an atmosphere of argon (50 mL/min), and a sample weight of 24.5 to 26.5 mg. The differential thermal analysis profile (μW) obtained is divided by the sample weight and thereby normalized (μW/mg). Thereafter, in the range of 0 to 100° C. in the differential thermal analysis profile, a region where the differential thermal analysis profile is horizontal is taken as a reference level of 0, and the height of exothermic peak from the reference level is measured.

In the present aspect, the number (number density) of Mg—Si clusters that have a relatively small size and are apt to dissolve during temperature increase in DSC, which are regarded as Mg—Si clusters not contributing to strength, is inhibited first. In the case where the number of such Mg—Si clusters that are apt to dissolve during temperature increase in DSC increases upon BH, the number (number density) of Mg—Si clusters that have a relatively large size and are less apt to dissolve during temperature increase in DSC, which are regarded as contributive to strength, in turn decreases upon an artificial age hardening treatment, making it impossible to increase strength after BH. Specifically, an increase in 0.2% proof stress of 100 MPa or greater and a strength (0.2% proof stress) after BH of 200 MPa or greater cannot be obtained, although this depends on the BH conditions.

Because of this, in the present aspect, the peak height of an endothermic peak A in a temperature range of 150 to 230° C., as an endothermic peak corresponding to the dissolution of Mg—Si clusters that are apt to dissolve during temperature increase in DSC and do not contribute to strength, is regulated (reduced) to 8 μW/mg or less (inclusive of 0 μW/mg). Consequently, that the endothermic peak in the temperature range of 150 to 230° C. has a peak height of 8 μW/mg indicates a critical number density which is permissible with respect to the adverse influence on strength of the Mg—Si clusters having a relatively small size and not contributing to strength. Although a sheet in which such Mg—Si clusters having a relatively small size and not contributing to strength are absent (i.e., the number density thereof is 0) is difficult to produce due to limitations on its production, the present aspect includes such the case. Because of this, the feature in which the peak height of the endothermic peak A is 8 μW/mg or less involves the case of 0 μW/mg, in which such Mg—Si clusters having a relatively small size and not contributing to strength are absent.

Meanwhile, in the present aspect, Mg—Si clusters which have a relatively large size and are less apt to dissolve during temperature increase in DSC and which contribute to strength are yielded in a large amount to improve the BH response. Because of this, the peak height of an exothermic peak B in a temperature range of 240 to 255° C., which corresponds to the formation of Mg—Si clusters that contribute to strength, is heightened (increased) to 20 μW/mg or more. Consequently, that the exothermic peak in the temperature range of 240 to 255° C. has a peak height of 20 μW/mg indicates a minimum value of the number density of Mg—Si clusters having a relatively large size and contributing to strength, the minimum value being necessary for obtaining the improvement in BH response which is addressed by the present aspect (an increase in 0.2% proof stress of 100 MPa or greater and a 0.2% proof stress after BH of 200 MPa or greater) though it differs by the BH condition. Hence, the higher the number density, the better, and the larger (higher) the peak height of the exothermic peak in the temperature range of 240 to 255° C., the better. However, in view of limitations on sheet production, an upper limit thereof is about 80 μW/mg.

FIG. 1 shows DSCs of three kinds of aluminum alloy sheet in Examples which will be given later, i.e., Invention Example 8 and Comparative Example 9 in Table 2 and Comparative Example 25 in Table 3. Invention Example 8 is indicated by a thick continuous line, Comparative Example 9 is indicated by a dotted line and Comparative Example 25 is indicated by a dot-and-dash line.

In FIG. 1, the DSC of Comparative Example 9 has an endothermic peak A in the temperature range of 150 to 230° C., which has a peak height exceeding (larger than) 8 μW/mg as shown in Table 2, which will be given later, showing that the number density of Mg—Si clusters having a relatively small size and not contributing to strength is too high. Meanwhile, the exothermic peak B in the temperature range of 240 to 255° C. also has a peak height as high (large) as 20 μW/mg or more, showing that the number density of Mg—Si clusters having a relatively large size and contributing to strength is also high. However, since the number density of the Mg—Si clusters having a relatively small size and not contributing to strength is too high, the adverse influences thereof are too greater. Therefore, the desired BH response (an increase in 0.2% proof stress of 100 MPa or greater and a 0.2% proof stress after BH of 200 MPa or greater) cannot be obtained.

The DSC of Comparative Example 25 in FIG. 1 has an endothermic peak A in the temperature range of 150 to 230° C., which has a peak height as low (small) as 8 μW/mg or less as shown in Table 2, which will be given later, showing that the number density of Mg—Si clusters having a relatively small size and not contributing to strength is low. Meanwhile, the exothermic peak B in the temperature range of 240 to 255° C. also has a peak height as low (small) as less than 20 μW/mg, showing that the number density of Mg—Si clusters having a relatively large size and contributing to strength is also too low. Because of this, the desired BH response (an increase in 0.2% proof stress of 100 MPa or greater and a 0.2% proof stress after BH of 200 MPa or greater) cannot be obtained.

In contrast, the DSC of Invention Example 8 in FIG. 1 has an endothermic peak A in the temperature range of 150 to 230° C., which has a peak height as low (small) as 8 μW/mg or less as shown in Table 2, which will be given later, showing that the number density of Mg—Si clusters having a relatively small size and not contributing to strength is low. Meanwhile, the exothermic peak B in the temperature range of 240 to 255° C. has a peak height as high (large) as 20 μW/mg or more, showing that the number density of Mg—Si clusters having a relatively large size and contributing to strength is high. Because of this, the desired BH response (an increase in 0.2% proof stress of 100 MPa or greater and a 0.2% proof stress after BH of 200 MPa or greater) is obtained.

(Production Process)

Next, a process for producing the aluminum alloy sheet according to the present aspect is explained. The aluminum alloy sheet according to the present aspect is produced through production steps which themselves are common or known, by subjecting, after casting, an aluminum alloy slab having the 6000-series component composition to a homogenizing heat treatment, hot rolling and cold rolling to obtain a given sheet thickness, followed by a refining treatment such as a solution quenching treatment.

However, for obtaining the structure specified with a DSC according to the present aspect, during those production steps, the average cooling rate in a quenching treatment after a solution treatment is controlled and in addition, the conditions for a preliminary aging treatment after the quenching treatment are regulated so as to be in a preferred range, as will be described later. With respect to other steps, there are preferred conditions for obtaining the structure specified with a DSC according to the present aspect. Unless such preferred conditions are employed, it is difficult to obtain the structure specified with a DSC according to the present aspect.

(Melting and Casting Cooling Rate)

First, in melting and casting steps, an aluminum alloy molten metal that has been melted and regulated so as to have a component composition within the 6000-series composition range is cast by a suitably selected ordinary melting and casting method, such as a continuous casting method or a semi-continuous casting method (DC casting method). Here, it is preferable that the average cooling rate, during the casting, from the liquidus temperature to the solidus temperature is as high (quick) as possible at 30° C./min or greater.

In the case where such temperature (cooling rate) control in a high-temperature range during casting is not performed, the cooling rate in this high-temperature range is inevitably low. When an average cooling rate in the high-temperature range is low as the above, the amount of crystals yielded coarsely in the temperature range of this high-temperature range is increased and also unevenness in the size and amount of the crystals along the width direction and thickness direction of the slab is increased. As a result, the basic mechanical properties, such as strength and elongation, which are a prerequisite for the 6000-series aluminum alloy sheet are reduced.

(Homogenizing Heat Treatment)

Next, the aluminum alloy slab obtained by casting is subjected to a homogenizing heat treatment prior to hot rolling. The purpose of this homogenizing heat treatment (soaking treatment) is to homogenize the structure, that is, to eliminate segregation within the grains in the structure of the slab. The conditions are not particularly limited so long as the purpose is achieved therewith, and the treatment may be an ordinary one conducted once or in one stage.

A homogenizing heat treatment temperature is suitably selected from the range of 500° C. or more and lower than the melting point, and a homogenizing time is suitably selected from the range of 4 hours and longer. In the case where the homogenizing temperature is low, the segregation within grains cannot be sufficiently eliminated, and these act as starting points for fracture, resulting in decreases in stretch flangeability and bendability. Thereafter, hot rolling may be started immediately. Alternatively, hot rolling may be started after holding and cooling to an appropriate temperature.

After the homogenizing heat treatment has been performed, cooling to room temperature may be performed so that the average cooling rate in the range of 300° C. to 500° C. is 20 to 100° C./hr, followed by reheating to 350° C. to 450° C. at an average heating rate of 20 to 100° C./hr to start hot rolling in this temperature range.

In the cases when the average cooling rate after the homogenizing heat treatment and the reheating rate conducted thereafter do not satisfy those conditions, the possibility of forming coarse Mg—Si compounds increases, resulting in decreases in the basic mechanical properties, such as strength and elongation, that are a prerequisite for the 6000-series aluminum alloy sheet before exhibiting the effect of Sn.

(Hot Rolling)

The hot rolling is constituted of a slab rough rolling step and a finish rolling step in accordance with the thickness of the plate to be rolled. In these rough rolling step and finish rolling step, rolling mills such as a reverse type and a tandem type are suitably used.

In the cases when the hot-rolling (rough-rolling) start temperature exceeds the solidus temperature, burning occurs and, hence, the hot rolling itself is difficult to carry out. Meanwhile, in the cases when the hot-rolling start temperature is lower than 350° C., the hot-rolling load is too high, rendering the hot rolling itself difficult. Consequently, the hot-rolling start temperature is preferably in the range of 350° C. to the solidus temperature, more preferably in the range of 400° C. to the solidus temperature.

(Annealing of the Hot-Rolled Plate)

Annealing (rough annealing) before cold rolling is not always necessary for the hot-rolled plate. However, it may be performed in order to further improve properties such as formability by making the grains smaller and optimizing the texture.

(Cold Rolling)

In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolled sheet (including a coil) having a desired final sheet thickness. However, for making the grains even smaller, it is desirable that the total cold rolling ratio should be 60% or greater regardless of the number of passes.

(Solution Treatment and Quenching Treatment)

After the cold rolling, a solution treatment is performed, followed by a treatment for quenching to room temperature. The solution and quenching treatments may be a heating and a cooling performed on an ordinary continuous heat treatment line, and are not particularly limited. However, from the standpoint of obtaining a sufficient solid-solution amount of each element and because it is desirable that the grains should be finer as stated above, it is desirable that the treatments should be conducted under such conditions of heating at a heating rate of 5° C./sec or greater to a solution treatment temperature which is 520° C. or higher and lower than the melting temperature, and then holding for 0.1 to 10 seconds.

From the standpoint of suppressing the formation of coarse intergranular compounds that reduce the formability and hem workability, it is desirable that the average cooling rate from the solution treatment temperature to the quenching stop temperature, which is room temperature, should be 3° C./s or greater. In the case where the rate of cooling to room temperature after the solution treatment is too low, coarse Mg—Si and elemental Si are yielded during the cooling, resulting in impaired formability. In addition, the solid-solution amount after the solution treatment is reduced, resulting in a decrease in BH response. In order to secure that cooling rate, means such as air cooling with fans or water cooling with mist or spray or by immersion, etc. and conditions therefor are selected and used for the treatment for quenching to room temperature.

(Preliminary Aging Treatment: Reheating Treatment)

After having thus undergone the solution treatment and the subsequent quenching treatment to be cooled to room temperature, the cold-rolled sheet is subjected to a preliminary aging treatment (reheating treatment) within 1 hour. In the case where the room-temperature holding period from termination of the treatment for quenching to room temperature to initiation of the preliminary ageing treatment (initiation of heating) is too long, the small Mg—Si clusters that do not contribute to strength are yielded in a large amount as clusters that are apt to dissolve upon room-temperature aging, making it difficult to suppress a peak height of the endothermic peak in the temperature range of 150 to 230° C. to 8 μW/mg or less. Consequently, the shorter the room-temperature holding period, the better. The solution and quenching treatments and the reheating treatment may be consecutively performed so that there is substantially no pause therebetween, and a lower limit of the period is not particularly determined.

In this preliminary aging treatment, the rate of temperature increase to a preliminary-aging temperature and the period of holding in a preliminary-aging temperature range are regulated. It is preferable that the temperature increase rate, of these, should be as high (quick) temperature increase rate as possible at 1° C./s or higher and preferably 5° C./s or higher, for suppressing the formation of the small Mg—Si clusters not contributing to strength. In the case where the temperature increase rate is less than 1° C./s, Mg—Si clusters that are apt to dissolve during temperature increase in DSC and that do not contribute to strength are yielded in a large amount, making it difficult to suppress a peak height of the endothermic peak in the temperature range of 150 to 230° C. to 8 μW/mg or less.

The temperature and holding period in the preliminary aging treatment is a holding in a temperature range of 60 to 120° C. for 10 hr or more and 40 hr or less. Here, the holding in the temperature range of 60 to 120° C. may be a heat treatment in which the temperature is constant or the temperature is sequentially changed by temperature increase or annealing, within that temperature range. In short, the temperature may be continuously changed by annealing, temperature increase, etc., so long as it is held in a temperature range of 60 to 120° C. for that period of 10 hr or more and 40 hr or less.

In the case where the preliminary-aging temperature is lower than 60° C. or the holding period is less than 10 hr, the formation of precipitate nuclei is insufficient and this is prone to result in a DSC in which an exothermic peak in the range of the exothermic peak B in the temperature range of 240 to 255° C. has a peak temperature higher than 255° C. This means that the amount of Mg—Si clusters having a relatively large size and contributing to strength decreases, and it becomes impossible to regulate the exothermic peak B in the temperature range of 240 to 255° C. so as to have a peak height as high (large) as 20 μW/mg or more. As a result, the BH response decreases.

Meanwhile, in the case where the preliminary-aging temperature exceeds 120° C. or the holding period exceeds 40 hr, precipitate nuclei are yielded in too large an amount in this preliminary aging treatment. Because of this, the Mg—Si clusters having a relatively large size and contributing to strength in turn decreases, making it impossible to make the exothermic peak B in the temperature range of 240 to 255° C. have a peak height as high (large) as 20 μW/mg or more, in a DSC. Consequently, this also results in a decrease in BH response. In addition, strength during forming is too high.

Namely, unless the preliminary aging treatment is regulated to fall within these preferable conditions, it is difficult to attain a 0.2% proof stress, in forming of automotive panels, reduced to 110 MPa or less and a 0.2% proof stress after BH of 200 MPa or greater.

The second aspect of the present invention will be explained below in detail with respect to each requirement.

(Chemical Component Composition)

First, the chemical component composition of the Al—Mg—Si (hereinafter referred to also as 6000-series) aluminum alloy sheet according to the present aspect is explained below. The 6000-series aluminum alloy sheet targeted by the present aspect, as, for example, the sheet for the automotive outer panels, is required to have various properties such as excellent formability, BH response, strength, weldability, and corrosion resistance. Consequently, such requirements are also met by means of the composition. In addition, in the present aspect, Sn is incorporated to suppress the room-temperature aging of the sheet after production, to reduce a 0.2% proof stress in the panel forming to 110 MPa or less and to reduce a yield ratio to less than 0.50. Thus, the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures, is improved. Simultaneously therewith, a 0.2% proof stress after bake hardening of 190 MPa or greater is rendered possible by means of the composition.

In order to satisfy such requirements, the aluminum alloy sheet according to the present aspect has a composition which includes, in terms of mass %, Mg: 0.3 to 1.0%, Si: 0.5 to 1.5% and Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable impurities. All the content indicated in % of the elements means that in mass %. In this description, percentage on mass basis (mass %) is the same as percentage on weight basis (wt %). With respect to the content of a chemical component, there are cases where “X % or less (exclusive of 0%)” is expressed by “more than 0% and X % or less”.

In the present aspect, elements other than the Mg, Si and Sn are impurities or elements which may be contained, and may have contents (permissible amounts) on levels of the elements in accordance with the AA or JIS standards, etc.

Namely, for the same reasons as in the first aspect, inclusion of such other elements shown below is permissible in the present aspect within the range of equal to or less than the upper limits specified below, which are in accordance with the AA or JIS standards or the like.

Specifically, the aluminum alloy sheet may further contain one kind or two or more kinds selected from the group consisting of Fe: 1.0% or less (exclusive of 0%), Mn: 0.4% or less (exclusive of 0%), Cr: 0.3% or less (exclusive of 0%), Zr: 0.3% or less (exclusive of 0%), V: 0.3% or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%), Cu: 0.4% or less (exclusive of 0%), Ag: 0.2% or less (exclusive of 0%), and Zn: 1.0% or less (exclusive of 0%), within those ranges, in addition to the basic composition shown above.

In the cases where these elements are contained, the content of Cu is preferably 0.3% or less, because Cu is prone to impair the corrosion resistance when contained in a large amount. Mn, Fe, Cr, Zr, and V are prone to yield relatively coarse compounds when contained in large amounts, and are prone to impair the hem workability (hem bendability), which is addressed by the present aspect. Consequently, the content of Mn is preferably 0.35% or less, and the content of each of Cr, Zr and V is preferably 0.2% or less and more preferably 0.1% or less.

The content range of each element and the purposes and permissible amount thereof in the 6000-series aluminum alloy are explained below in order.

Si: 0.5 to 1.5%

Si, together with Mg, is an essential element for obtaining the strength (proof stress) required as automotive panels by forming aging precipitates which contribute to an improvement in strength, during an artificial aging treatment such as a baking treatment, and thus exhibiting an age hardenability. Furthermore, solute Si is an element that improves the work hardenability, and Si, when present in a solid-solution state, has the effect of lowering the yield ratio, which is a ratio between tensile strength and yield strength [(0.2% proof stress)/(tensile strength)], less than 0.50.

In the case where the content of Si is too low, the amount of precipitates after an artificial age hardening treatment is too small, resulting in a decrease in the strength increase due to baking and resulting in a reduced amount of solute Si and hence in too large a yield ratio exceeding 0.50. Meanwhile, in the case where the content of Si is too high, the Si forms coarse crystals with impurity Fe, etc., resulting in a considerable decrease in formability such as bendability. In addition, too high Si contents increases not only the strength just after sheet production but also the room-temperature aging amount after the production, thereby increases the strength before forming too much, and reduces the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures. Consequently, the content of Si is regulated so as to be in the range of 0.5 to 1.5%.

For attaining an excellent age hardenability in a baking treatment performed at a lower temperature for a shorter period after forming into panels, it is preferable to employ a 6000-series aluminum alloy composition in which Si/Mg is 1.0 or larger in terms of mass ratio so that Si has been incorporated further excessively relative to the Mg than in the so-called excess-Si type.

Mg: 0.3 to 1.0%

Mg, together with Si, is also an important element for forming the atom aggregates specified in the present aspect. It is an essential element for obtaining the proof stress required as panels by forming, together with the Si, aging precipitates which contribute to an improvement in strength, during an artificial aging treatment such as a baking treatment, and thus exhibiting an age hardenability. Furthermore, solute Mg, like Si, is an element that improves the work hardenability, and Mg, when present in a solid-solution state, has the effect of lowering the yield ratio, which is a ratio between tensile strength and yield strength [(0.2% proof stress)/(tensile strength)], to less than 0.50.

In the case where the content of Mg is too low, the amount of precipitates after an artificial age hardening treatment is too small, resulting in a decrease in the strength increase due to baking and resulting in a reduced amount of solute Mg and hence in too large a yield ratio exceeding 0.50. Meanwhile, in the case where the content of Mg is too high, the Mg forms coarse crystals with impurity Fe, etc., resulting in a considerable decrease in formability such as bendability. In addition, too high Mg contents increases not only the strength just after sheet production but also the room-temperature aging amount after the production, thereby increases the strength before forming, and reduces the formability into automotive panels or the like, which are particularly problematic in face strains thereof, in automotive panel structures. Consequently, the content of Mg is regulated so as to be in the range of 0.3 to 1.0%.

Sn: 0.005 to 0.3%

Sn has the important effect of attaining both an increase in BH response and a reduction in yield ratio by reducing the volume proportion of atom aggregates which enhance the 0.2% proof stress in panel forming, even when the solid-solution amount of Mg+Si, which will be described later, is increased. In general, for increasing the solid-solution amount of Mg+Si, it is effective to increase the amount of Mg and/or Si to be contained in the sheet. However, such increase in Mg and Si contents in the sheet result not only in an increase in 0.2% proof stress in panel forming but also in an increase in the volume proportion of atom aggregates which inhibit yield ratio reduction. It has hence been difficult, with any of conventional compositions or production processes, to attain all of an increase in BH response, a reduction in proof stress and a reduction in yield ratio. In contrast, according to the present aspect, atom aggregates that inhibit yield ratio reduction can be diminished even when the solid-solution amount of Mg+Si is increased to enhance the BH response, by incorporating Sn in an amount within the range shown above. Thus, all of an increase in BH response, a reduction in proof stress, and a reduction in yield ratio can be attained.

Sn, at room temperature, has the effect of capturing (trapping) atomic holes to thereby inhibit room-temperature diffusion of Mg and Si and inhibit the strength increase at room temperature (room-temperature age hardening), and during the forming of the sheet into panels, improving the press formability including hem workability, drawability and punch stretch formability (hereinafter, this press formability is referred to also as hem workability as a representative). During an artificial aging treatment of the panels, such as a baking treatment, it releases the captured holes and hence in turn enhances the diffusion of Mg and Si, thereby enhancing the BH response.

In the case where the content of Sn is lower than 0.005%, the effects described above, i.e., the effect of lowering the volume proportion of atom aggregates that inhibit yield ratio reduction, even when the solid-solution amount of Mg+Si is increased, and thereby attaining both an increase in BH response and a reduction in yield ratio and the effect of suppressing room-temperature age hardening. Meanwhile, in the case where the content of Sn is higher than 0.3%, the Sn segregates at grain boundaries and it is prone to cause intergranular cracks. A preferred lower limit of the content of Sn is 0.01%. An upper limit of the content of Sn is preferably 0.2%, more preferably 0.1% and further preferably 0.06%

(Solid-Solution Amount Mg and Si)

The composition described above is employed. Furthermore, in the present aspect, the total solid-solution amount of Mg and Si contained in the sheet (solid-solution amount of Mg+Si) is increased and ensured so as to be in a specified range of 1.0 mass % or more and 2.0 mass % or less, in order to enhance the BH response. In the case where the solid-solution amount of Mg+Si is less than 1.0 mass %, the BH response cannot be ensured even when the composition described above is employed. The larger the solid-solution amount of Mg+Si, the more the BH response improves. However, due to the composition and production described above, there are limitations on the contents and solid-solution amount of Mg and Si. In addition, too high amount of solid-solution pose a problem in that the volume proportion of the atom aggregates increases to result in increases in proof stress and yield ratio during panel forming. An upper limit thereof is hence 2.0 mass %.

The solid-solution amount of Mg+Si in a sheet is measured by dissolving a sample of the sheet to be examined, by a residue extraction method using hot phenol, separating the solid/liquid by filtration with a filter having a mesh of 0.1 μm, and regarding the total content of Mg and Si in the separated solution as the solid-solution amount of Mg+Si.

The residue extraction method with hot phenol is performed specifically in the following manner. First, phenol is introduced into a decomposition flask and heated, and each sheet sample to be examined is then transferred to this decomposition flask and decomposed by heating. Subsequently, benzyl alcohol is added thereto, followed by performing a suction filtration with the filter, thereby separating the solid/liquid by filtration. The solution separated is quantitatively analyzed to determine the total content of Mg and Si therein. For this quantitative analysis, use is suitably made of atomic absorption spectrometry (AAS), inductively coupled plasma optical emission spectroscopy (ICPOES) or the like. For the suction filtration, a membrane filter having a diameter of 47 mm and having a mesh (capture particle diameter) of 0.1 μm is used as stated above. This examination and a calculation are made with respect to three samples obtained from total of three portions, i.e., one central portion in the sheet-width direction of the test sheet and two portions located respectively at both ends of the sheet-width-direction from the central portion. The solid-solution amount of Mg+Si (mass %) of these samples are averaged.

(Aggregates of Atoms)

The composition and structure described above are employed. In addition, in the present aspect, the structure of the 6000-series aluminum alloy sheet is regulated in the size distribution of aggregates of Mg and Si atoms observed with a three-dimensional atom probe filed ion microscope, in order to reduce a yield ratio to less than 0.50 and to ensure BH response. Thus, both an increase in BH response and a reduction in yield ratio are attained not only by the effects of the Sn but also by regulating the atom aggregates (clusters) present in the structure of the sheet.

(Definition of Aggregate of Atoms)

In the present aspect, one which satisfies some conditions (requirements) specified through an examination and analysis based on the principle of three-dimensional atom probe field ion microscope is defined as an aggregate of atoms, as described in the section Effects. Specifically, defined as an aggregate of atoms is one which satisfies some conditions (requirements) specified in the present aspect with respect to a three-dimensional structure of atoms (three-dimensional atom map) obtained by a reconstruction through analysis from the flight times and positions of atoms of the sheet which have temporarily ionized in a high electric field (electric-field evaporation) with a three-dimensional atom probe field ion microscope.

Consequently, the aggregates of atoms specified in the present aspect are not real atom aggregates (clusters) exist in 6000-series aluminum alloy sheets, such as ones observed by directly examining the structure of a sheet as such with a high-magnification TEM (transmission electron microscope) as in Patent Document 1. However, they correlate deeply with the state in which the real atom aggregates (clusters) exist in 6000-series aluminum alloy sheets, such as ones directly observed with a high-magnification TEM. Because of this, even if the examination of atom aggregates in the present aspect is indirect or simulative, the atom aggregates satisfactorily correlate with the state in which those real atom aggregates (clusters) exist, the state considerably affecting a reduction in yield ratio and an increase in BH response. It hence provides a measure for ensuring a reduction in yield ratio and an increase in BH response by means of the structure (atom aggregates).

The sheet to be examined here is a 6000-series aluminum alloy sheet which has undergone refining such as a solution treatment and a quenching treatment and which has undergone neither press forming nor a bake hardening treatment. The structure of arbitrary central portion in the sheet-thickness direction of this sheet is examined with a three-dimensional atom probe field ion microscope.

(Requirements which the Atom Aggregates should Meet)

Requirements (prerequisites) for being defined (regarded) as atom aggregates in the present aspect are explained below.

The requirements which the atom aggregates in the present aspect should meet are the same as in Patent Documents 2 and 3. First, either or both of an Mg atom and an Si atom are contained by a total of 10 pieces or more. Although an upper limit on the number of the Mg atom and/or Si atom included in the atom aggregate is not particularly determined, an upper limit of the number of the Mg atom and/or Si atom included in the atom aggregate is about 10,000 in view of limitations on production.

Furthermore, the ones are regarded as atom aggregates, in which when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less. The distance therebetween of 0.75 nm is a value which has been experimentally fixed in order that atom aggregates in each of which the distance between atoms of Mg and/or Si is short and which each have a size that considerably affects a reduction in yield ratio and an increase in BH response, and the volume proportion thereof are specified with satisfactory reproducibility, although the technical meaning of the value has not been fully elucidated.

The atom aggregates specified in the present aspect mostly are ones each including both of Mg atom and Si atom. However, they may include one which includes Mg atoms but contains no Si atom and one which includes Si atoms but contains no Mg atom. Furthermore, they need not to be composed of Mg atoms and/or Si atoms only, and it is highly probable that Al atoms are contained besides these.

Moreover, depending on the component composition of the aluminum alloy sheet, there inevitably are cases where atoms contained as alloying elements or impurities, such as Fe, Mn, Cu, Cr, Zr, V, Ti, Zn, Ag, etc., are contained in the atom aggregates and those other atoms are counted by the 3DAP analysis. However, even in the cases when such other atoms (derived from alloying elements or impurities) are contained in the atom aggregates, they are on a low level as compared with the total number of Mg atoms and Si atoms. Consequently, even in the cases when such other atoms are contained in the atom aggregates, those meet the limitations (requirements) function as the atom aggregates according to the present aspect like the atom aggregates composed of Mg atoms and/or Si atoms only. Thus, the atom aggregates specified in the present aspect may contain any other atoms so long as they satisfy the limitations.

The wording “when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less” means that each of all the Mg atoms and Si atoms present in each aggregate of atoms has at least one Mg atom or Si atom therearound within a distance of 0.75 nm or less.

In the limitation on the distance between atoms in the atom aggregates according to the present aspect, when any atom of the Mg atom and the Si atom contained therein is used as a reference, each of all the distances between the atom as the reference and all atoms among other atoms adjacent thereto may not be 0.75 nm or less, and on the contrary, each of them all may be 0.75 nm or less. In other words, other Mg atom or Si atom may be adjacent at a distance exceeding 0.75 nm, and it is sufficient that in the periphery of a specific Mg atom or Si atom (serving as a reference), at least one Mg atom or Si atom is present which satisfies the specified distance (spacing).

In the case where there is one adjacent other Mg atom or Si atom which satisfies the specified distance, the number of Mg atom and/or Si atom which satisfy the requirement concerning distance and which should be counted is 2, including the specific Mg atom or Si atom (serving as a reference). Meanwhile, in the case where there are two adjacent other Mg atoms and/or Si atoms which satisfy the specified distance, the number of Mg atoms and/or Si atoms which satisfy the requirement concerning distance and which should be counted is 3, including the specific Mg atom or Si atom (serving as a reference).

(Regulation of the Atom Aggregates)

First, in the present aspect, as the total volume of atom aggregates which satisfy the given requirements explained above, including the number of Mg atoms and/or Si atoms and the distance between atoms, the total volume ΣVi is determined by summing up the volumes of the individual atom aggregates Vi (=4/3πr_(G) ³) calculated from the Guinier radii r_(G) of the individual atom aggregates each regarded as a sphere. Then, the average volume proportion of this total volume ΣVi to the volume V_(Al) of the aluminum alloy sheet measured with the three-dimensional atom probe field ion microscope, (ΣVi/V_(Al))×100, is regulated so as to be in the range of 0.3 to 1.5%.

Furthermore, in the present aspect, in addition to the regulation of the volume proportion of atom aggregates, the average volume proportion of the total volume ΣVi_(1.5 or more), which is the total volume of atom aggregates each having the Guinier radius r_(G) of 1.5 nm or larger among the atom aggregates satisfying those requirements, to the total volume of the atom aggregates ΣVi, (ΣVi_(or more)/ΣVi), is regulated so as to be in the range of 20 to 70%. Namely, the individual atom aggregates which each satisfy the requirements are divided at a Guinier radius r_(G) of 1.5 nm, and the average volume proportion of the total volume ΣVi_(1.5 or more) obtained by summing up the volumes V_(1.5 or more) of the individual atom aggregates each having a Guinier radius r_(G) of 1.5 nm or larger to the total volume of the atom aggregates V, (ΣVi_(1.5 or more)/ΣVi)×100, is regulated so as to be in the range of 20 to 70%.

Guinier radius r_(G) is determined in the following manner. The individual atom aggregates which each satisfy the requirements are each regarded as a sphere, and the largest of values of radius of gyration I_(g) of each atom aggregate is taken as the radius of gyration I_(g) of the atom aggregate. The Guinier radius r_(G) is a radius obtained by converting this radius of gyration I_(g) by using the equation which will be described later. The definition of Guinier radius and the method for calculation thereof which will be described later are known by Patent Documents 2 and 3.

Due to those structure regulations in combination with the compositional regulation, it is possible to make the 6000-series aluminum alloy sheet have, in automotive-panel forming, a 0.2% proof stress reduced to 110 MPa or less and a yield ratio reduced to less than 0.50 and further have a 0.2% proof stress after BH of 190 MPa or greater.

In the case where the average volume proportion of atom aggregates which satisfy those requirements, (ΣVi/V_(Al))×100, is less than 0.3%, the absolute number of relatively large atom aggregates having a Guinier radius r_(G) of 1.5 nm or larger and effective for an increase in BH response and a reduction in yield ratio is insufficient. Because of this, even when the composition is satisfied, it is impossible to attain the increase in BH response and reduction in yield ratio. Meanwhile, in the case where the average volume proportion (ΣVi/V_(Al))×100 exceeds 1.5%, the number of atom aggregates which satisfy the requirements, including the distance between atoms being 0.75 nm or less, is too large, making it impossible to attain reduction in 0.2% proof stress and reduction in yield ratio in panel forming.

Furthermore, also in the case where the average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 of relatively large atom aggregates having a Guinier radius r_(G) of 1.5 nm or larger and effective for an increase in BH response and a reduction in yield ratio is less than 20%, the absolute number of these atom aggregates is insufficient and a reduction in yield ratio cannot be attained even when the composition is satisfied or even when the average volume proportion of atom aggregates which satisfy those requirements satisfies the limitation. Meanwhile, the larger the number or proportion of relatively large atom aggregates having a Guinier radius r_(G) of 1.5 nm or larger, the easier the attainment of a reduction in yield ratio. However, it is difficult, from the standpoint of production, to increase the average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 beyond 70%, and this 70% is determined as an upper limit in view of limitations on production.

(Principle of Measurement with 3DAP and Method of the Measurement Therewith)

The principle of a measurement with a 3DAP and a method of the measurement therewith are also known from Patent Documents 1 to 3. The 3DAP (three-dimensional atom probe) is configured of a field ion microscope (FIM) and a time-of-flight mass spectrometer attached thereto. Due to such configuration, this is a local analyzer in which individual atoms in a metal surface can be observed with the field ion microscope and these atoms can be identified by time-of-flight mass spectrometry. Furthermore, since the 3DAP can simultaneously analyze the kinds and positions of atoms emitted from a sample, it can be an exceedingly effective means for analyzing the structure of atom aggregates. Because of this, it is used as a known technique for, for example, structural analysis of magnetic recording films, electronic devices, steel materials or the like, as stated above. Recently, it is used for determination or the like of atom aggregates in the structure of an aluminum alloy sheet, as described above.

The 3DAP utilizes the phenomenon called electric-field evaporation, in which atoms of a sample themselves are ionized in a high electrical field. When a high voltage necessary for causing atoms of a sample to undergo electric-field evaporation is applied to the sample, atoms are ionized from the sample surface, and they pass through the probe hole and reach a detector.

This detector is a position sensitive detector, which performs mass spectrometry for individual ions (identification of elements that are kinds of atom) and measures the time of flight of each ion to the detector and which can thereby simultaneously determine the positions detected (atom structure positions). Consequently, the 3DAP can simultaneously measure the positions and atom kinds of atoms present at the tip of the sample, and hence has the feature of being able to three-dimensionally reconstitute and observe the structure of atoms present in the tip of the sample. In addition, because electric-field evaporation takes place in order from the surface of the sample tip, the depth-direction distribution of atoms from the sample tip can be examined with atomic-level resolution.

Since the 3DAP utilizes a high electric field, the sample to be analyzed is required to have high electroconductivity, like metals, etc., and the shape of the sample is generally required to be an ultrafine needle shape having a tip diameter of about 100 nm or less. Because of this, a sample is taken from, for example, a central portion in a sheet-thickness direction of an aluminum alloy sheet to be examined, and this sample is cut with a precise cutting device and electropolished to produce a sample for analysis which has an ultrafine needle-shaped tip portion. A measuring method is as follows. “LEAP 3000”, manufactured by Imago Scientific Instruments Corp., is, for example, used, and a high-pulse voltage on the order of 1 kV is applied to the aluminum alloy sheet sample having a tip formed in a needle-shape, thereby continuously ionize millions of atoms from the sample tip. The ions are detected by the position sensitive detector. Mass spectrometry of each ion (identification of the element that is the kind of atom) is conducted on the basis of the time of flight from the emission of the individual ion from the sample tip, which is caused by the pulse-voltage application, to the arrival at the detector.

Furthermore, the feature in which the electric-field evaporation takes place regularly in order form the surface of the sample tip is utilized to suitably give a depth-direction coordinate to a two-dimensional map which shows ion arrival sites, and analysis software “IVAS” is used to conduct a three-dimensional mapping (construction of three-dimensional structure of atoms: atom map). Thus, a three-dimensional atom map for the sample tip can be obtained.

This three-dimensional atom map is further processed by a maximum separation method, which is a method for defining atoms belonging to a precipitate or to an atom aggregate, to analyze aggregates of atoms (atom aggregates). In this analysis, the number of either or both of Mg atom and Si atom (ten pieces or more in total), the distance (spacing) between adjacent Mg atoms and/or Si atoms, and the number of Mg atoms and/or Si atoms which have the specific narrow spacing (0.75 nm or less) are given as parameters.

Then, atom aggregates which satisfy conditions in which either or both of an Mg atom and an Si atom are contained by a total of 10 pieces or more and, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, are defined as atom aggregates according to the present aspect. In addition, the dispersion state in which atom aggregates according to the definition is evaluated, and the number density of atom aggregates is quantified by examining three or more samples and averaging the measured values in terms of average density per 1 m³ (pieces/m³).

That is, a maximum radius of gyration I_(g) when each of the atom aggregates being examined is regarded as a sphere by using the analysis software originally specific to the 3DAP is acquired by using the following formula of Math. 1.

$\begin{matrix} {l_{g} = \sqrt{\frac{\sum\limits_{i = 1}^{n}\left\lbrack {\left( {x_{i} - \overset{\_}{x}} \right)^{2} + \left( {y_{i} - \overset{\_}{y}} \right)^{2} + \left( {z_{i} - \overset{\_}{z}} \right)^{2}} \right\rbrack}{n}}} & \left\lbrack {{Math}.\mspace{14mu} 1} \right\rbrack \end{matrix}$

In the formula of Math. 1, I_(g) represents a radius of gyration automatically calculated by the software specific to the three-dimensional atom probe field ion microscope. x, y and z respectively represent an x axis, a y axis and a z axis which are invariable in the measuring layout of the three-dimensional atom probe field ion microscope. x_(i), y_(i) and z_(i) respectively represent the lengths of the x axis, y axis and z axis, and are spacial coordinates for the Mg atoms and/or Si atoms which constitute the atom aggregate. “x bar” and the like in which “-” is placed on the top of each of “x”, “y” and “z” also represent the lengths of the x, y and z axes, but are barycentric coordinates for the atom aggregate. n represents the number of Mg atoms and/or Si atoms which constitute the atom aggregate.

Next, a maximum of the radius of gyration I_(g) of each of the individual atom aggregates is taken as the radius of gyration I_(g) of the atom aggregate, and converted to a Guinier radius r_(G) by using the relationship r_(G)=√(5/3)·I_(g) of the following formula of Math. 2. This Guinier radius r_(G) obtained by the conversion is regarded as the radius of the atom aggregate.

$\begin{matrix} {r_{G} = {\sqrt{\frac{5}{3}} \cdot l_{g}}} & \left\lbrack {{Math}.\mspace{14mu} 2} \right\rbrack \end{matrix}$

On the basis of this, the volumes Vi (=4/3πr_(G) ³) of the individual atom aggregates which satisfy those requirements are summed up to determine the total volume ΣVi. Meanwhile, the volume of the needle-shaped sample which has undergone the electric-field evaporation (i.e., which has disappeared due to electric-field evaporation) is taken as the volume V_(Al) of the aluminum alloy sheet measured with the three-dimensional atom probe field ion microscope, and the average volume proportion of the total volume of the atom aggregates thereto, (ΣVi/V_(Al))×100, is determined. Furthermore, the average volume proportion of the total volume ΣVi_(1.5 or more) of atom aggregates each having a Guinier radius r_(G) of 1.5 nm or larger to the total volume V of the atom aggregates, (ΣVi_(1.5 or more)/ΣVi)×100, is also determined. The measurement of each average volume proportion of atom aggregates with the 3DAP is made on arbitrary ten regions of central portions in the sheet-thickness direction in the 6000-series aluminum alloy sheet which has undergone the refining, and the measured values (calculated values) thereof are averaged.

The calculation formula for calculating the radius of an atom aggregate and the methods for measuring and converting the radius of gyration I_(g) for Guinier radius r_(G) are based on quotations from M. K. Miller: Atom Probe Tomography, (Kluwer Academic/Plenum Publishers, New York, 2000), p. 184. Calculation formulae for the radius of an atom aggregate are described in many documents other than this. For example, “(2) Three-dimensional Atom Probe Analysis” on page 140 of “Microstructural Evolution in Low Alloy Steels under High Dose Ion Irradiation” (Katsuhiko Fujii, Koji Fukuya, Tadakatsu Ohkubo, Kazuhiro Hono, et al.) describes including the formula of Math. 1 and the formula for conversion to Guinier radius r_(G) (in this document, however, the symbol of the radius of gyration I_(g) is described as r_(G)).

(Efficiency of Atom Detection by 3DAP)

Currently, the efficiency of the detection of atoms by the 3DAP is about 50% at the most with respect to the ionized atoms, and the remaining atoms cannot be detected. In the cases when the efficiency of the detection of atoms by the 3DAP changes considerably due to, for example, an improvement in the future, there is a possibility that the results of measurements, with a 3DAP, of the average number density (pieces/μm³) of each of atom aggregates of the sizes specified in the present aspect may vary. Consequently, from the standpoint of conducting the measurements with reproducibility, it is preferable that the efficiency of the detection of atoms with a 3DAP should be kept approximately constant at about 50%.

(Production Process)

Next, a process for producing the aluminum alloy sheet according to the present aspect is explained. The aluminum alloy sheet according to the present aspect is produced through production steps which themselves are common or known, by subjecting, after casting, an aluminum alloy slab having the 6000-series component composition to a homogenizing heat treatment, hot rolling and cold rolling to obtain a given sheet thickness, followed by a refining treatment such as a solution quenching treatment.

However, for obtaining the structure including the atom aggregates specified with a 3DAP according to the present aspect, during those production steps, the average cooling rate in a quenching treatment after a solution treatment is controlled and in addition, the conditions for a preliminary aging treatment after the quenching treatment are regulated so as to be in a preferred range, as will be described later. With respect to other steps, there are preferred conditions for obtaining the structure specified in the present aspect. Unless such preferred conditions are employed, it is difficult to obtain the structure according to the present aspect.

(Melting and Casting Cooling Rate)

First, in melting and casting steps, an aluminum alloy molten metal that has been melted and regulated so as to have a component composition within the 6000-series composition range is cast by a suitably selected ordinary melting and casting method, such as a continuous casting method or a semi-continuous casting method (DC casting method). Here, it is preferable that the average cooling rate, during the casting, from the liquidus temperature to the solidus temperature is as high (quick) as possible at 30° C./min or greater.

In the case where such temperature (cooling rate) control in a high-temperature range during casting is not performed, the cooling rate in this high-temperature range is inevitably low. When an average cooling rate in the high-temperature range is low as the above, the amount of crystals yielded coarsely in the temperature range of this high-temperature range is increased and also unevenness in the size and amount of the crystals along the width direction and thickness direction of the slab is increased. As a result, the basic mechanical properties, such as strength and elongation, which are a prerequisite for the 6000-series aluminum alloy sheet are reduced.

(Homogenizing Heat Treatment, Hot Rolling, Annealing of Hot-Rolled Plate, Cold Rolling, and Solution and Quenching Treatments)

Subsequently, the aluminum alloy slab obtained by casting is subjected to the treatments of a homogenizing heat treatment, hot rolling, annealing of the hot-rolled plate (according to need), cold rolling, and solution and quenching treatments in the same manners as in the first aspect. The conditions for these treatments are the same as in the first aspect, and explanations thereon are omitted here.

(Preliminary Aging Treatment: Reheating Treatment)

After having thus undergone the solution treatment and the subsequent quenching treatment to be cooled to room temperature, the cold-rolled sheet is subjected to a preliminary aging treatment (reheating treatment) within a period which is as short as possible and is up to 1 hour (60 minutes).

In the case where the room-temperature holding period from termination of the quenching treatment to room temperature to initiation of the preliminary aging treatment (initiation of heating) is too long and exceeds 1 hour, it becomes impossible to regulate the total volume of atom aggregates to 1.5% or less in terms of average volume proportion, the atom aggregates satisfying the requirements concerning the number of Mg atoms and/or Si atoms and the distance between atoms. In addition, relatively large clusters are less apt to be yielded, making it impossible to increase the average volume proportion of atom aggregates each having a Guinier radius r_(G) of 1.5 nm or larger to the atom aggregates which satisfy the requirements to 20% or higher. As a result, the BH response decreases, and a reduction in yield ratio is also difficult. Consequently, the shorter the room-temperature holding period, the better. The solution and quenching treatments and the reheating treatment may be consecutively performed so that there is substantially no pause therebetween, and a lower limit of the period is not particularly determined.

In this preliminary aging treatment, the rate of temperature increase to a preliminary-aging temperature and the period of holding in a preliminary-aging temperature range are regulated. It is preferable that the temperature increase rate, of these, should be as high (quick) temperature increase rate as possible at 1° C./s or higher, preferably 5° C./s or higher, for suppressing the formation of small atom aggregates not contributing to strength. In the case where the temperature increase rate is less than 1° C./s, small atom aggregates not contributing to strength are yielded in a large amount, making it impossible to increase the average volume proportion of atom aggregates each having a Guinier radius r_(G) of 1.5 nm or larger to the atom aggregates which satisfy the requirements to 20% or higher. As a result, the BH response decreases, and a reduction in yield ratio is also difficult.

The temperature and holding period in the preliminary aging treatment is a holding in a temperature range of 60 to 120° C. for 10 hr or more and 40 hr or less. Here, the holding in the temperature range of 60 to 120° C. may be a heat treatment in which the temperature is constant or the temperature is sequentially changed by temperature increase or annealing, within that temperature range. In short, the temperature may be continuously changed by annealing, temperature increase, etc., so long as it is held in a temperature range of 60 to 120° C. for that period of 10 hr or more and 40 hr or less.

In the case where the preliminary-aging temperature is lower than 60° C. or the holding period is less than 10 hr, the formation of precipitate nuclei is insufficient, making it impossible to increase the average volume proportion of atom aggregates each having a Guinier radius r_(G) of 1.5 nm or larger to the atom aggregates which satisfy the requirements to 20% or higher. As a result, the BH response decreases.

Meanwhile, in the case where the preliminary-aging temperature exceeds 120° C. or the holding period exceeds 40 hr, precipitate nuclei are yielded in too large an amount in this preliminary aging treatment. Because of this, the amount of atom aggregates having a relatively large size and contributing to strength decreases. As a result, the average volume proportion of atom aggregates which satisfy the requirements increases beyond 1.5%, making it impossible to enable the sheet in forming to have a yield ratio reduced to less than 0.50.

Namely, unless the preliminary aging treatment is regulated to fall within these preferable conditions, it is difficult to produce a sheet which has, in automotive-panel forming, a 0.2% proof stress reduced to 110 MPa or less and a yield ratio reduced to less than 0.50 and further has a 0.2% proof stress after BH of 190 MPa or greater.

EXAMPLES

The present invention will be explained below in more detail by reference to Examples. However, the present invention should not, of course, be construed as being limited by the following Examples, and can be suitably modified unless the modifications depart from the gist of the present invention described hereinabove and hereinafter. All such modifications are included in the technical range of the present invention.

Examples According to the First Aspect

Next, Examples according to the first aspect of the present invention are explained. 6000-series aluminum alloy sheets were individually produced so as to differ in the structure specified with a DSC in the present aspect, by changing the conditions for a preliminary aging treatment performed after solution and quenching treatments. After a holding at room temperature for 30 days after the production of the sheets, BH response (bake hardenability), As proof stress as an index of press formability and hem workability as bendability are examined and evaluated.

For individually producing the structure specified with a DSC, the 6000-series aluminum alloy sheets having the compositions shown in Table 1 was produced by variously changing conditions such as the average cooling rate in the quenching treatment after a solution treatment and the temperature and holding period in the subsequent preliminary aging treatment as shown in Tables 2 and 3. With respect to the indications of the contents of elements within Table 1, a value of the element expressed by a blank indicates that the content is below a detection limit.

Specific conditions for aluminum alloy sheet production were as follows. Slabs of aluminum alloys respectively having the compositions shown in Table 1 were commonly produced through casting by the DC casting method. In this casting, the average rate of cooling from the liquidus temperature to the solidus temperature was set at 50° C./min in common with all the Examples. Subsequently, the slabs were subjected to a soaking treatment of 540° C.×6 hours performed in one stage only, followed by initiation of hot rough rolling at that temperature, in common with all the Examples. Thereafter, they were hot-rolled, in the succeeding finish rolling, to a thickness of 3.5 mm to obtain hot-rolled sheets, in common with all the Examples. The hot-rolled aluminum alloy sheets were subjected to rough annealing of 500° C.×1 minute and then to cold rolling at a processing rate of 70% without performing intermediate annealing during the cold-rolling passes, to obtain cold-rolled sheets having a thickness of 1.0 mm, in common with all the Examples.

Furthermore, the cold-rolled sheets were each continuously subjected to a refining treatment (T4) with continuous type heat treatment facilities while unwinding and winding each sheet, in common with all the Examples. Specifically, a solution treatment was performed by heating at an average rate of heating to 500° C. of 10° C./sec and holding for seconds after the temperature reached a target temperature of 560° C., followed by cooling to room temperature by water cooling or air cooling so as to result in the average cooling rates shown in Tables 2 and 3. After this cooling and after the subsequent required periods shown in Table 2 at room temperature, a preliminary aging treatment was performed by using an atmospheric furnace and an oil bath and using the temperature increase rates, reached temperatures, average cooling rates, and holding periods shown in Tables 2 and 3. As for the cooling after this preliminary aging treatment, water cooling or gradual cooling (natural cooling) was conducted in order to change the average rate of cooling.

From the final product sheets which each had been allowed to stand at room temperature for 30 days after the refining treatment, test sheets (blanks) were cut out and the DSC and properties of the test sheets were examined and evaluated. The results thereof are shown in Table 3.

(DSC)

The structure in each of ten portions of the central portion in the sheet-thickness direction in each test sheet was examined for the DSC. In the DSC (differential scanning calorimetry curves) of this sheet, as for the average value for these ten portions, the peak height (W/mg) of an endothermic peak in the temperature range of 150 to 230° C. as an endothermic peak corresponding to the dissolution of Mg—Si clusters not contributing to strength and the peak height (μW/mg) of an exothermic peak in the temperature range of 240 to 255° C. as an exothermic peak corresponding to the formation of Mg—Si clusters contributing to strength were determined.

The differential thermal analysis of each of the measurement portions in each test sheet was performed under the same conditions including a test apparatus of DSC220G, manufactured by Seiko Instruments Inc., a reference substance of aluminum, a sample container made of aluminum, temperature increase conditions of 15° C./min, an atmosphere of argon (50 mL/min), and a sample weight of 24.5 to 26.5 mg. The differential thermal analysis profile (μW) obtained was divided by the sample weight and thereby normalized (μW/mg). Thereafter, in the range of 0 to 100° C. in the differential thermal analysis profile, a region where the differential thermal analysis profile was horizontal was taken as a reference level of 0, and the height of exothermic peak from the reference level was measured. The results thereof are shown in Tables 2 and 3.

(Bake hardenability)

The test sheets which had been allowed to stand at room temperature for 30 days after the refining treatment were each examined for 0.2% proof stress (As proof stress) as a mechanical property through a tensile test. Furthermore, these test sheets were aged at room temperature for 30 days, subsequently subjected to an artificial age hardening treatment of 170° C.×20 minutes (after BH), and then examined for 0.2% proof stress (proof stress after BH) through a tensile test, in common with the test sheets. The BH response of each test sheet was evaluated on the basis of the difference between these 0.2% proof stresses (increase in proof stress).

With respect to the tensile test, No. 5 specimens (25 mm×50 mmGL×sheet thickness) according to JIS Z2201 were cut out of each sample sheet to perform the tensile test at room temperature. Here, the tensile direction of each specimen was set so as to be perpendicular to the rolling direction. The tensile rate was set at 5 mm/min until the 0.2% proof stress and at 20 mm/min after the proof stress. The number N of examinations for mechanical property was 5, and an average value therefor was calculated. With respect to the specimens to be examined for the proof stress after BH, a 2% pre-strain as a simulation of sheet press forming was given to the specimens by the tensile tester, followed by performing the BH treatment.

(Hem Workability)

Hem workability was evaluated only with respect to the test sheets which had been allowed to stand at room temperature for 30 days after the refining treatment. In the test, strip-shaped specimens having a width of 30 mm were used and subjected to 90° bending at an inward bending radius of 1.0 mm with a down flange. Thereafter, an inner plate having a thickness of 1.0 mm was nipped, and the specimen was subjected, in order, to pre-hem working in which the bent part was further bent inward to approximately 130° and flat-hem working in which the bent part was further bent inward to 180° and the end portion was brought into close contact with the inner plate.

The surface state, such as the occurrence of rough surface, a minute crack or a large crack, of the bent part (edge bent part) of the flat hem was visually examined and visually evaluated on the basis of the following criteria. In the following criteria, ratings of 0 to 2 are on an acceptable level, and ratings of 3 and larger are unacceptable.

0, no crack and no rough surface; 1, slight rough surface; 2, deep rough surface; 3, minute surface crack; 4, linearly continued surface crack.

Invention Examples Nos. 0, 1, 8, and 13 in Table 2 and Nos. 16 to 24 in Table 3, which employ alloys Nos. 0 to 12 shown in Table 1, each is not only within the component composition range according to the present aspect and has been produced under conditions within preferred ranges but also has undergone the refining treatment, including the solution quenching treatment and the preliminary aging treatment, under preferred conditions. Because of this, these Invention Examples satisfy the DSC requirements specified in the present aspect, as shown in Tables 2 and 3. That is, in the DSCs of these sheets, the endothermic peak in the temperature range of 150 to 230° C. as an endothermic peak corresponding to the dissolution of Mg—Si clusters not contributing to strength had a peak height of 8 μW/mg or less, while the exothermic peak in the temperature range of 240 to 255° C. as an exothermic peak corresponding to the formation of Mg—Si clusters contributing to strength had a peak height of 20 μW/mg or larger.

As a result, the Invention Examples each show excellent BH response although the bake hardening is performed after the refining treatment and subsequent room-temperature aging and is a treatment conducted at a low temperature for a short period of time. Furthermore, as shown in Table 3, even after the refining treatment and subsequent room-temperature aging, they each have a relatively low As proof stress and hence show excellent press formability into automotive panels or the like and excellent hem workability. That is, the Invention Examples, even when having undergone an automotive-baking treatment after room-temperature aging, were able to exhibit not only high BH response with a 0.2% proof stress difference of 100 MPa or greater and a 0.2% proof stress after BH of 170 MPa or greater but also press formability with an As 0.2% proof stress of 110 MPa or less and satisfactory bendability.

In contrast, Comparative Examples 2 to 7, 9 to 13, 14, and 15 in Table 2, which employed alloy example 1, 2 or 3 in Table 1 like Invention Examples, each have the preliminary aging treatment conditions outside the preferred ranges, as shown in Table 2. As a result, they each gave a DSC which was outside the range specified in the present aspect, and show enhanced room-temperature aging and, in particular, a relatively high As proof stress after 30-day room-temperature holding, as compared with the Invention Examples having the same alloy composition. Because of this, they are poor in press formability into automotive panels or the like and in hem workability and are poor also in BH response.

In Comparative Example 2, the average cooling rate in the quenching treatment to room temperature performed after the solution treatment is too low. Because of this, the exothermic peak B in the temperature range of 240 to 255° C. has a peak height as low (small) as less than 20 μW/mg, although the endothermic peak A in the temperature range of 150 to 230° C. has a peak height of 8 μW/mg or less, showing that the number density of Mg—Si clusters having a relatively large size and contributing to strength is low. This is because due to the low cooling rate in the quenching treatment to room temperature, coarse Mg₂Si and elemental Si were yielded during the cooling. Neither the desired press formability with an As 0.2% proof stress of 110 MPa or less nor satisfactory bendability is obtained. In addition, the BH response is low.

In Comparative Examples 3 and 9, the period from the quenching treatment to room temperature after the solution treatment to the preliminary aging treatment (initiation of heating) is too long. Because of this, Mg—Si clusters that are apt to dissolve during temperature increase in DSC and do not contribute to strength have been yielded in a large amount, and the endothermic peak A in the temperature range of 150 to 230° C. has a peak height higher (larger) than 8 μW/mg, as shown in FIG. 1. Meanwhile, the exothermic peak B in the temperature range of 240 to 255° C. has a peak height which also is as high (large) as 20 μW/mg or more, showing that the number density of Mg—Si clusters having a relatively large size and contributing to strength is high. However, since the number density of Mg—Si clusters having a relatively small size and not contributing to strength is too high, the adverse influences thereof are too greater. Therefore, the desired press formability with an As 0.2% proof stress of 110 MPa or less and satisfactory bendability cannot be obtained. In addition, the BH response is low.

In Comparative Examples 4 and 10, the temperature increase rate in the preliminary aging treatment is too low. Because of this, Mg—Si clusters that are apt to dissolve during temperature increase in DSC and do not contribute to strength have undesirably been yielded in a large amount, and the endothermic peak A in the temperature range of 150 to 230° C. has a peak height higher (larger) than 8 μW/mg, as shown in FIG. 1. Meanwhile, the exothermic peak B in the temperature range of 240-255° C. has a peak height which also is as high (large) as 20 μW/mg or more, showing that the number density of Mg—Si clusters having a relatively large size and contributing to strength is high. However, since the number density of Mg—Si clusters having a relatively small size and not contributing to strength is too high, the adverse influences thereof are too greater. Therefore, the desired press formability with an As 0.2% proof stress of 110 MPa or less and satisfactory bendability cannot be obtained. In addition, the BH response is low.

In Comparative Examples 5, 11 and 14, the period of holding in the range of 60 to 120° C. in the preliminary aging treatment is 1 hour, which is too short. Because of this, Mg—Si clusters that are apt to dissolve during temperature increase in DSC and do not contribute to strength have been yielded in a large amount, and the endothermic peak A in the temperature range of 150 to 230° C. has a peak height higher (larger) than 8 μW/nag, as shown in FIG. 1. Meanwhile, the exothermic peak B in the temperature range of 240 to 255° C. has a peak height which also is as high (large) as 20 μW/mg or more, showing that the number density of Mg—Si clusters having a relatively large size and contributing to strength is high. However, since the number density of Mg—Si clusters having a relatively small size and not contributing to strength is too high, the adverse influences thereof are too greater. Therefore, the desired press formability with an As 0.2% proof stress of 110 MPa or less and satisfactory bendability cannot be obtained. In addition, the BH response is low.

In Comparative Examples 6, 12 and 15, the period of holding in the range of 60 to 120° C. in the preliminary aging treatment is 48 hours, which is too long. Because of this, the exothermic peak B in the temperature range of 240 to 255° C. has a peak height as low (small) as less than 20 μW/mg, showing that the number density of Mg—Si clusters having a relatively large size and contributing to strength is low. As a result, the desired press formability with an As 0.2% proof stress of 110 MPa or less and satisfactory bendability cannot be obtained. In addition, the BH response is low.

In Comparative Example 7, the reached temperature in the preliminary aging treatment is 130° C., which exceeds the upper limit of 120° C. and is too high. Because of this, the amount of Mg—Si clusters having a relatively large size and contributing to strength has decreased, and the exothermic peak B in the temperature range of 240 to 255° C. thus has a peak height as low (small) as less than 20 μW/mg, showing that the number density of the Mg—Si clusters having a relatively large size and contributing to strength is low. As a result, the BH response is low and the As 0.2% proof stress exceeds 110 MPa and is too high, and press formability and satisfactory bendability cannot be obtained, too.

Comparative Examples 25 to 34 in Table 3 have been produced within preferred ranges, including the conditions for the preliminary aging treatment. However, since they employed alloys Nos. 13 to 22 shown in Table 1, the contents of Mg and Si, which are essential elements, therein are outside the ranges according to the present aspect or the content of impurity elements therein is too high. Because of this, these Comparative Examples 24 to 33 each show, in particular, a relatively high As proof stress after 30-day room-temperature holding as compared with the Invention Examples, as shown in Table 3, and hence are poor in press formability into automotive panels or the like and in hem workability or are poor in BH response.

Comparative Example 25 is alloy 13 shown in Table 1, in which the Si content is too low.

Comparative Example 26 is alloy 14 shown in Table 1, in which the Si content is too high.

Comparative Example 27 is alloy 15 shown in Table 1, in which the Sn content is too low.

Comparative Example 28 is alloy 16 shown in Table 1, in which the Sn content is too high and cracking occurred during the hot rolling, making the sheet production impossible.

Comparative Example 29 is alloy 17 shown in Table 1, in which the Fe content is too high.

Comparative Example 30 is alloy 18 shown in Table 1, in which the Mn content is too high.

Comparative Example 31 is alloy 19 shown in Table 1, in which the Cr content and Ti content are too high.

Comparative Example 32 is alloy 20 shown in Table 1, in which the Cu content is too high.

Comparative Example 33 is alloy 21 shown in Table 1, in which the Zn content is too high.

Comparative Example 34 is alloy 22 shown in Table 1, in which the Zr content and V content are too high.

Those results of the Examples establish that, for improving formability and BH response after room-temperature aging, it is necessary that all the requirements concerning composition and DSC specified in the present aspect should be satisfied.

TABLE 1 Alloy Chemical components of Al—Mg—Si alloy sheet (mass %; remainder, Al) No. Mg Si Sn Fe Mn Cr Zr V Ti Cu Zn Ag 0 0.64 0.99 0.040 1 0.58 0.90 0.050 0.2 2 0.40 0.82 0.039 0.2 0.05 0.12 3 0.39 1.18 0.058 0.2 0.2 0.01 4 0.34 1.50 0.097 0.2 0.64 5 0.54 1.31 0.053 0.2 0.22 6 0.55 0.79 0.197 0.2 0.12 7 0.45 0.89 0.042 0.2 0.65 0.05 8 0.64 1.15 0.027 0.2 0.05 0.05 9 1.47 0.53 0.110 0.2 0.3 0.01 10 0.71 1.00 0.055 0.2 0.05 11 0.47 1.23 0.002 0.7 0.6 12 0.55 0.87 0.050 0.2 0.2 0.1 0.1 13 1.53 0.21 0.046 0.2 14 0.40 2.10 0.042 0.2 15 0.58 1.02 0.002 0.2 16 0.60 1.09 0.455 0.2 17 0.38 0.80 0.051 1.3 18 0.65 1.04 0.046 0.2 1.21 0.01 19 0.51 0.80 0.057 0.2 0.44 0.08 20 0.36 0.79 0.044 0.2 1.28 21 0.48 1.01 0.052 0.2 1.23 22 0.49 0.94 0.055 0.2 0.4 0.4 * Field in which the value for the element is blank indicates below detection limit.

TABLE 2 Solution quenching treatment Preliminary aging Solution Required period Period of treatment Average to preliminary Temperature Reached holding Average cooling Alloy No. temperature cooling rate aging increase rate temperature at 60 to 120° C. rate Classification No. in Table 1 ° C. ° C./s min ° C./s ° C. hr ° C./s Inv. Ex. 0 0 540 100 5 20 100 12 100 Inv. Ex. 1 1 540 100 5 20 100 12 100 Com. Ex. 2 1 540 1 5 20 100 12 100 Com. Ex. 3 1 540 100 120 20 100 12 100 Com. Ex. 4 1 540 100 5 0.1 100 12 100 Com. Ex. 5 1 540 100 5 20 100 1 100 Com. Ex. 6 1 540 100 5 20 100 48 100 Com. Ex. 7 1 540 100 5 20 130 12 100 Inv. Ex. 8 2 540 100 5 20 90 12 100 Com. Ex. 9 2 540 100 80 20 90 12 100 Com. Ex. 10 2 540 100 5 0.1 90 12 100 Com. Ex. 11 2 540 100 5 20 90 3 100 Com. Ex. 12 2 540 100 5 20 90 48 100 Inv. Ex. 13 3 540 100 5 20 100 16 0.05 Com. Ex. 14 3 540 100 5 5 100 3 100 Com. Ex. 15 3 540 100 5 5 100 45 0.02 Structure of aluminum alloy sheet after 30-day room-temperature holding Properties of aluminum alloy after Differential scanning calorimetry curve 30-day room-temperature holding Height of Height of 0.2% proof endothermic exothermic Exothermic peak B As 0.2% stress after Proof Alloy No. peak A peak B temperature proof stress BH stress increase Classification No. in Table 1 μW/mg μW/mg ° C. MPa MPa MPa Hem workability Inv. Ex. 0 0 1.5 48.2 251 103 220 117 1 Inv. Ex. 1 1 0.9 45.3 251 94 226 132 1 Com. Ex. 2 1 2.8 18.5 254 127 209 82 3 Com. Ex. 3 1 13.0 60.3 257 133 201 68 3 Com. Ex. 4 1 8.2 54.1 256 114 206 92 2 Com. Ex. 5 1 8.6 53.3 256 93 183 90 1 Com. Ex. 6 1 1.3 16.2 250 147 262 115 4 Com. Ex. 7 1 1.0 5.3 251 156 224 68 4 Inv. Ex. 8 2 0.6 35.4 251 88 206 118 1 Com. Ex. 9 2 12.3 60.3 254 135 200 65 3 Com. Ex. 10 2 8.3 63.2 256 112 205 93 2 Com. Ex. 11 2 8.8 58.9 256 90 179 89 1 Com. Ex. 12 2 0.8 15.5 250 144 261 117 4 Inv. Ex. 13 3 2.1 40.6 250 95 216 121 1 Com. Ex. 14 3 9.1 55.7 256 97 192 95 1 Com. Ex. 15 3 1.7 14.2 250 150 269 119 4

TABLE 3 Solution quenching treatment Preliminary aging Solution Required period Period of treatment Average to preliminary Temperature Reached holding Average cooling Alloy No. temperature cooling rate aging increase rate temperature at 60 to 120° C. rate Classification No. in Table 1 ° C. ° C./s min ° C./s ° C. hr ° C./s Inv. Ex. 16 4 540 50 5 20 100 12 100 Inv. Ex. 17 5 540 20 5 20 100 12 100 Inv. Ex. 18 6 540 100 15 20 100 12 100 Inv. Ex. 19 7 540 100 5 5 100 12 100 Inv. Ex. 20 8 540 100 5 3 100 12 100 Inv. Ex. 21 9 540 100 5 20 80 12 100 Inv. Ex. 22 10 540 100 5 20 100 8 100 Inv. Ex. 23 11 540 100 5 20 80 27 0.1 Inv. Ex. 24 12 540 100 5 20 70 32 0.1 Com. Ex. 25 13 540 100 5 20 100 12 100 Com. Ex. 26 14 540 100 5 20 100 12 100 Com. Ex. 27 15 540 100 5 20 100 12 100 Com. Ex. 28 16 cracking occurred during hot rolling Com. Ex. 29 17 540 100 5 20 100 12 100 Com. Ex. 30 18 540 100 5 20 100 12 100 Com. Ex. 31 19 540 100 5 20 100 12 100 Com. Ex. 32 20 540 100 5 20 100 12 100 Com. Ex. 33 21 540 100 5 20 100 12 100 Com. Ex. 34 22 540 100 5 20 100 12 100 Structure of aluminum alloy sheet after 30-day room-temperature holding Properties of aluminum alloy after Differential scanning calorimetry curve 30-day room-temperature holding Height of Height of As 0.2% 0.2% proof endothermic exothermic Exothermic peak B proof stress after Proof Alloy No. in peak A peak B temperature stress BH stress increase Classification No. Table 1 μW/mg μW/mg ° C. MPa MPa MPa Hem workability Inv. Ex. 16 4 2.3 50.2 251 101 213 112 2 Inv. Ex. 17 5 2.2 54.2 251 104 222 118 2 Inv. Ex. 18 6 5.6 42.9 253 103 207 104 1 Inv. Ex. 19 7 2.4 45.0 251 92 208 116 2 Inv. Ex. 20 8 3.0 68.4 252 107 221 114 1 Inv. Ex. 21 9 3.4 47.1 250 98 201 103 1 Inv. Ex. 22 10 4.3 64.9 249 105 215 110 2 Inv. Ex. 23 11 2.5 68.1 248 104 219 115 2 Inv. Ex. 24 12 3.6 57.3 251 84 195 111 1 Com. Ex. 25 13 3.6 6.8 258 78 126 48 1 Com. Ex. 26 14 10.2 53.6 251 125 207 82 4 Com. Ex. 27 15 1.7 19.5 268 133 255 122 3 Com. Ex. 28 16 cracking occurred during hot rolling Com. Ex. 29 17 1.3 38.2 251 106 209 103 4 Com. Ex. 30 18 2.2 37.2 251 114 212 98 4 Com. Ex. 31 19 4.0 36.0 251 103 209 106 4 Com. Ex. 32 20 4.3 36.6 250 135 246 111 4 Com. Ex. 33 21 2.3 34.5 251 112 205 93 4 Com. Ex. 34 22 1.6 39.0 251 108 212 104 4

Next, Examples according to the second aspect of the present invention are explained. 6000-series aluminum alloy sheets were individually produced so as to differ in the structure specified in the present aspect, by changing the conditions for a preliminary aging treatment performed after solution and quenching treatments. After a holding at room temperature for 30 days after the production of the sheets, BH response (bake hardenability), As proof stress as an index of press formability and hem workability as bendability are examined and evaluated.

For individually producing the structure, the 6000-series aluminum alloy sheets having the compositions shown in Table 4 was produced by variously changing conditions such as the average cooling rate in the quenching treatment after a solution treatment and the temperature and holding period in the subsequent preliminary aging treatment as shown in Tables 5 and 6. With respect to the indications of the contents of elements within Table 4, a value of the element expressed by a blank indicates that the content is below a detection limit.

Specific conditions for aluminum alloy sheet production were as follows. Slabs of aluminum alloys respectively having the compositions shown in Table 4 were commonly produced through casting by the DC casting method. In this casting, the average rate of cooling from the liquidus temperature to the solidus temperature was set at 50° C./min in common with all the Examples. Subsequently, the slabs were subjected to a soaking treatment of 540° C.×6 hours performed in one stage only, and were then reheated to 500° C. to initiate hot rough rolling, in common with all the Examples. Thereafter, they were hot-rolled, in the succeeding finish rolling, to a thickness of 3.5 mm to obtain hot-rolled sheets, in common with all the Examples. The hot-rolled aluminum alloy sheets were subjected to rough annealing of 500° C.×1 minute and then to cold rolling at a processing rate of 70% without performing intermediate annealing during the cold-rolling passes, to obtain cold-rolled sheets having a thickness of 1.0 mm, in common with all the Examples.

Furthermore, the cold-rolled sheets were each continuously subjected to a refining treatment (T4) with continuous type heat treatment facilities while unwinding and winding each sheet, in common with all the Examples. Specifically, a solution treatment was performed by heating at an average rate of heating to 500° C. of 10° C./sec and holding for seconds after the temperature reached a target temperature of 560° C., followed by cooling to room temperature by water cooling or air cooling so as to result in the average cooling rates shown in Tables 5 and 6. After this cooling and after the subsequent required periods shown in Table 2 at room temperature, a preliminary aging treatment was performed by using an atmospheric furnace and an oil bath and using the temperature increase rates, reached temperatures, average cooling rates, and holding periods shown in Tables 5 and 6. As for the cooling after this preliminary aging treatment, water cooling or gradual cooling (natural cooling) was conducted in order to change the average rate of cooling.

From the final product sheets which each had been allowed to stand at room temperature for 30 days after the refining treatment, test sheets (blanks) were cut out and the structure and properties of the test sheets were examined and evaluated. The results thereof are shown in Tables 5 and 6.

(Structure)

The solid-solution amount of Mg+Si in the sheet, volume proportions of atom aggregates determined with a three-dimensional atom probe field ion microscope, etc. were determined through measurements and analysis by the measuring methods described above. In Tables 5 and 6, the average volume proportions (%) of atom aggregates determined with a three-dimensional atom probe field ion microscope are abbreviated to “average volume proportions of atom aggregates determined with 3DAP (%)”.

In the “average volume proportions of atom aggregates” in Tables 5 and 6, the average volume proportion (ΣVi/V_(Al))×100 of the total volume ΣVi of atom aggregates which satisfied the requirements specified in the present aspect to the volume V_(Al) of the needle-shaped sample which had undergone electric-field evaporation was determined (in Tables 2 and 3, it is referred to as Σvi/V_(Al)×100). Furthermore, the average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 of the total volume ΣVi_(1.5 or more) of atom aggregates having a Guinier radius r_(G) of 1.5 nm or larger to the total volume ΣVi of the atom aggregates was also determined (in Tables 5 and 6, it is referred to as ΣVi_(1.5 or more)/ΣVi×100).

(Bake Hardenability)

The test sheets which had been allowed to stand at room temperature for 30 days after the refining treatment were each examined for 0.2% proof stress (As proof stress) as a mechanical property through a tensile test. Furthermore, these test sheets were aged at room temperature for 30 days, subsequently subjected to an artificial age hardening treatment of 170° C.×20 minutes (after BH), and then examined for 0.2% proof stress (proof stress after BH) through a tensile test, in common with the test sheets. The BH response of each test sheet was evaluated on the basis of the difference between these 0.2% proof stresses (increase in proof stress).

With respect to the tensile test, No. 5 specimens (25 mm×50 mmGL×sheet thickness) according to JIS Z2201 were cut out of each sample sheet to perform the tensile test at room temperature. Here, the tensile direction of each specimen was set so as to be perpendicular to the rolling direction. The tensile rate was set at 5 mm/min until the 0.2% proof stress and at 20 mm/min after the proof stress. The number N of examination for mechanical property was 5, and an average value therefor was calculated. With respect to the specimens to be examined for the proof stress after BH, a 2% pre-strain as a simulation of sheet press forming was given to the specimens by the tensile tester, followed by performing the BH treatment.

(Hem Workability)

Hem workability was evaluated only with respect to the test sheets which had been allowed to stand at room temperature for 7 days or 100 days after the refining treatment. In the test, strip-shaped specimens having a width of 30 mm were used and subjected to 90° bending at an inward bending radius of 1.0 mm with a down flange. Thereafter, an inner plate having a thickness of 1.0 mm was nipped, and the specimen was subjected, in order, to pre-hem working in which the bent part was further bent inward to approximately 130° and flat-hem working in which the bent part was further bent inward to 180° and the end portion was brought into close contact with the inner plate.

The surface state, such as the occurrence of rough surface, a minute crack, or a large crack, of the bent part (edge bent part) of the flat hem was visually examined and visually evaluated on the basis of the following criteria. In the following criteria, ratings of 0 to 2 are on an acceptable level, and ratings of 3 and larger are unacceptable.

0, no crack and no rough surface; 1, slight rough surface; 2, deep rough surface; 3, minute surface crack; 4, linearly continued surface crack.

Invention Examples Nos. 35, 36, 43, and 48 in Table 5 and Nos. 51 to 58 in Table 6, which employ alloys Nos. 23 to 34 shown in Table 4, each is not only within the component composition range according to the present aspect and has been produced under conditions within preferred ranges but also has undergone the refining treatment, including the solution quenching treatment and the preliminary aging treatment, under preferred conditions. Because of this, these Invention Examples satisfy the structure requirements specified in the present aspect, as shown in Tables 5 and 6. That is, the solid-solution amount of Mg+Si is 1.0 mass % or more and 2.0 mass % or less, the average volume proportion (ΣVi/V_(Al))×100 of the total volume ΣVi of atom aggregates satisfying the requirements specified in the present aspect to the volume V_(Al) of the needle-shaped sample which has undergone electric-field evaporation is in the range of 0.3 to 1.5%, and the average volume proportion ΣVi_(1.5 or more)/ΣVi)×100 of the total volume ΣVi_(1.5 or more) of atom aggregates having a Guinier radius r_(G) of 1.5 nm or larger to the total volume ΣVi of the atom aggregates is 20 to 70%.

As a result, the Invention Examples each show excellent BH response although the bake hardening is performed after the refining treatment and subsequent room-temperature aging and is a treatment conducted at a low temperature for a short period of time. Furthermore, as shown in Table 6, even after the refining treatment and subsequent room-temperature aging, they each have a relatively low As proof stress and a low yield ratio and hence show excellent press formability into automotive panels or the like and excellent hem workability.

That is, the Invention Examples, even when having undergone an automotive-baking treatment after room-temperature aging, were able to exhibit not only high BH response with a 0.2% proof stress difference of 100 MPa or greater and a 0.2% proof stress after BH of 190 MPa or greater but also press formability with an As 0.2% proof stress of 110 MPa or less and a low yield ratio of less than 0.50 and satisfactory bendability. Thus, they have succeeded in combining formability and bake hardenability and in attaining both an increase in BH response and a reduction in yield ratio.

In contrast, Comparative Examples 37 to 42, 44 to 47, 49, and 50 in Table 5, which employed alloy examples 24, 25 and 26 in Table 4 like Invention Examples, each have the preliminary aging treatment condition outside the preferred ranges, as shown in Table 5. As a result, either the solid-solution amount of Mg+Si or the average volume proportion (ΣVi/V_(Al))×100 or the average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 is outside the range specified in the present aspect. As a result, they show enhanced room-temperature aging and, in particular, a relatively high As proof stress or an increased yield ratio after 30-day room-temperature holding, as compared with the Invention Examples having the same alloy composition. Because of this, they are poor in press formability into automotive panels or the like and in hem workability or are poor in BH response. Thus, they have failed to combine formability and bake hardenability and to attain both an increase in BH response and a reduction in yield ratio.

In Comparative Example 37, the average cooling rate in the quenching treatment to room temperature performed after the solution treatment is too low. Because of this, coarse Mg—Si and elemental Si were yielded during the cooling, resulting in low formability. In addition, the solid-solution amount of after the solution treatment is low, and the average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 also is less than 20%. Furthermore, the BH response is also low.

In Comparative Examples 38 and 44, the period from the quenching treatment to room temperature after the solution treatment to the preliminary aging treatment (initiation of heating) is too long. Because of this, the average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 is less than 20% and the BH response is low. A reduction in yield ratio was also unable to be attained.

In Comparative Examples 39 and 45, the temperature increase rate in the preliminary aging treatment is too low. Because of this, the average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 was unable to be increased to 20% or higher, resulting in low BH response.

In Comparative Examples 40, 46 and 49, the period of holding in the range of 60 to 120° C. in the preliminary aging treatment is 1 hour, which is too short. Because of this, the formation of precipitate nuclei was insufficient, and the average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 was unable to be increased to 20% or higher, resulting in low BH response.

In Comparative Examples 41, 47 and 50, the period of holding in the range of 60 to 120° C. in the preliminary aging treatment is 48 to 45 hours, which is too long. Because of this, precipitate nuclei were yielded in too large an amount in the preliminary aging treatment. As a result, the amount of atom aggregates having a relatively large size and contributing to strength has decreased and the average volume proportion (ΣVi/V_(Al))×100 has increased beyond 1.5%, resulting in a failure in reducing the yield ratio of the sheet during forming to less than 0.50.

In Comparative Example 42, the reached temperature in the preliminary aging treatment is 130° C., which exceeds the upper limit of 120° C. and is too high. Because of this, precipitate nuclei were yielded in too large an amount in the preliminary aging treatment. As a result, the amount of atom aggregates having a relatively large size and contributing to strength has decreased and the average volume proportion (ΣVi/V_(Al))×100 has increased beyond 1.5%, resulting in too high an As proof stress and a failure in reducing the yield ratio of the sheet during forming to less than 0.50.

Comparative Examples 59 to 67 in Table 6 have been produced within preferred ranges, including the conditions for the preliminary aging treatment. However, since they employed alloys Nos. 35 to 43 shown in Table 4, the contents of Mg and Si, which are essential elements, therein are outside the ranges according to the present aspect or the content of impurity elements therein is too high. Because of this, Comparative Examples 59 to 67 each show, in particular, too high an As proof stress and too high a yield ratio after 30-day room-temperature holding as compared with the Invention Examples, as shown in Table 6, and hence are poor in press formability into automotive panels or the like and in hem workability or are poor in BH response.

Comparative Example 59 is alloy 35 shown in Table 4, in which the Si content is too low.

Comparative Example 60 is alloy 36 shown in Table 4, in which the Si content is too high.

Comparative Example 61 is alloy 37 shown in Table 4, in which the Sn content is too low.

Comparative Example 62 is alloy 38 shown in Table 4, in which the Sn content is too high and cracking occurred during the hot rolling, making the sheet production impossible.

Comparative Example 63 is alloy 39 shown in Table 4, in which the Fe content is too high.

Comparative Example 64 is alloy 40 shown in Table 4, in which the Mn content is too high.

Comparative Example 65 is alloy 41 shown in Table 4, in which the Cr content and Ti content are too high.

Comparative Example 66 is alloy 42 shown in Table 4, in which the Zn content is too high.

Comparative Example 67 is alloy 43 shown in Table 4, in which the Zr content and V content are too high.

Those results of the Examples establish that, for improving formability and BH response after room-temperature aging, it is necessary that all the requirements concerning composition and structure specified in the present aspect should be satisfied.

TABLE 4 Alloy Chemical components of Al—Mg—Si alloy sheet (mass %; remainder, Al) Classification No. Mg Si Sn Fe Mn Cr Zr V Ti Cu Zn Ag Inv. Ex. 23 0.64 0.99 0.040 24 0.58 0.90 0.050 0.2 25 0.40 0.82 0.039 0.2 0.05 0.12 26 0.39 1.18 0.058 0.2 0.16 0.01 27 0.36 1.23 0.084 0.2 0.33 28 0.54 1.31 0.053 0.2 0.22 29 0.55 0.79 0.197 0.2 0.12 30 0.45 0.93 0.040 0.2 0.35 0.05 31 0.64 1.15 0.027 0.2 0.05 0.05 32 0.71 0.72 0.055 0.2 0.03 33 0.47 1.23 0.005 0.7 0.6 34 0.55 0.87 0.050 0.2 0.2 0.1 0.1 Com. Ex. 35 0.77 0.45 0.046 0.2 36 0.40 2.10 0.042 0.2 37 0.58 1.02 0.002 0.2 38 0.60 1.09 0.455 0.2 39 0.38 0.80 0.051 1.3 40 0.53 0.98 0.046 0.2 0.78 0.01 41 0.51 0.80 0.057 0.2 0.44 0.08 42 0.48 1.01 0.052 0.2 1.23 43 0.49 0.94 0.055 0.2 0.4 0.4 * Field in which the value for the element is blank indicates below detection limit.

TABLE 5 Solution quenching treatment Preliminary aging Solution Required period Period treatment Average to preliminary Temperature Reached of holding at Average cooling Alloy No. in temperature cooling rate aging increase rate temperature 60 to 120° C. rate Classification No. Table 4 ° C. ° C./s min ° C./s ° C. hr ° C./s Inv. Ex. 35 23 540 100 5 20 100 12 100 Inv. Ex. 36 24 540 100 5 20 100 12 100 Com. Ex. 37 24 540 1 5 20 100 12 100 Com. Ex. 38 24 540 100 120 20 100 12 100 Com. Ex. 39 24 540 100 5 0.1 100 12 100 Com. Ex. 40 24 540 100 5 20 100 1 100 Com. Ex. 41 24 540 100 5 20 100 48 100 Com. Ex. 42 24 540 100 5 20 130 12 100 Inv. Ex. 43 25 540 100 5 20 90 12 100 Com. Ex. 44 25 540 100 80 20 90 12 100 Com. Ex. 45 25 540 100 5 0.1 90 12 100 Com. Ex. 46 25 540 100 5 20 90 3 100 Com. Ex. 47 25 540 100 5 20 90 48 100 Inv. Ex. 48 26 540 100 5 20 100 16 0.05 Com. Ex. 49 26 540 100 5 5 100 3 100 Com. Ex. 50 26 540 100 5 5 100 45 0.02 Structure of aluminum alloy sheet after 30-day room-temperature holding Average volume proportions of atom aggregates Properties of aluminum alloy sheet after Solid-solution determined 30-day room-temperature holding amount of with 3DAP Yield ratio 0.2% Mg and Si (%) As As 0.2% [(proof proof Proof Alloy Mg Si ΣVi/ ΣVi_(1.5 or more)/ tensile proof stress)/ stress stress No. in mass mass Mg + Si V_(Al) × ΣVi × strength stress (tensile after BH increase Hem Classification No. Table 4 % % mass % 100 100 MPa MPa strength)] MPa MPa workability Inv. Ex. 35 23 0.61 0.89 1.50 0.74 31 221 103 0.466 220 117 1 Inv. Ex. 36 24 0.55 0.80 1.35 0.55 26 207 94 0.455 226 132 1 Com. Ex. 37 24 0.35 0.48 0.83 0.58 8 251 127 0.506 209 82 3 Com. Ex. 38 24 0.55 0.80 1.35 1.54 9 253 133 0.527 201 68 3 Com. Ex. 39 24 0.55 0.80 1.35 1.15 15 234 114 0.488 206 92 2 Com. Ex. 40 24 0.55 0.80 1.35 0.48 10 211 93 0.441 183 90 1 Com. Ex. 41 24 0.55 0.80 1.35 2.45 45 274 147 0.536 262 115 4 Com. Ex. 42 24 0.55 0.80 1.35 2.73 42 274 156 0.568 224 68 4 Inv. Ex. 43 25 0.38 0.71 1.09 0.42 20 204 88 0.431 206 118 1 Com. Ex. 44 25 0.38 0.71 1.09 1.42 6 264 135 0.512 200 65 3 Com. Ex. 45 25 0.38 0.71 1.09 1.01 9 227 112 0.492 205 93 2 Com. Ex. 46 25 0.38 0.71 1.09 0.44 16 202 90 0.444 179 89 1 Com. Ex. 47 25 0.38 0.71 1.09 2.27 35 271 144 0.532 261 117 4 Inv. Ex. 48 26 0.37 1.02 1.39 0.55 32 207 95 0.458 216 121 1 Com. Ex. 49 26 0.37 1.02 1.39 0.65 17 215 97 0.452 192 95 1 Com. Ex. 50 26 0.37 1.02 1.39 2.59 51 282 150 0.531 269 119 4

TABLE 6 Solution quenching treatment Preliminary aging Solution Required period Period treatment Average cooling to preliminary Temperature Reached of holding at Average Alloy No. in temperature rate aging increase rate temperature 60 to 120° C. cooling rate Classification No. Table 4 ° C. ° C./s min ° C./s ° C. hr ° C./s Inv. Ex. 51 27 540 50 5 20 100 12 100 Inv. Ex. 52 28 540 20 5 20 100 12 100 Inv. Ex. 53 29 540 100 15 20 100 12 100 Inv. Ex. 54 30 540 100 5 5 100 12 100 Inv. Ex. 55 31 540 100 5 3 100 12 100 Inv. Ex. 56 32 540 100 5 20 100 8 100 Inv. Ex. 57 33 540 100 5 20 80 27 0.1 Inv. Ex. 58 34 540 100 5 20 70 32 0.1 Com. Ex. 59 35 540 100 5 20 100 12 100 Com. Ex. 60 36 540 100 5 20 100 12 100 Com. Ex. 61 37 540 100 5 20 100 12 100 Com. Ex. 62 38 cracking occurred during hot rolling Com. Ex. 63 39 540 100 5 20 100 12 100 Com. Ex. 64 40 540 100 5 20 100 12 100 Com. Ex. 65 41 540 100 5 20 100 12 100 Com. Ex. 66 42 540 100 5 20 100 12 100 Com. Ex. 67 43 540 100 5 20 100 12 100 Structure of aluminum alloy sheet after 30-day room-temperature holding Average volume proportions of atom Properties of aluminum alloy sheet after aggregates determined 30-day room-temperature holding with 3DAP As Yield ratio Solid-solution amount (%) As 0.2% [(proof 0.2% proof Proof Alloy of Mg and Si ΣVi_(1.5 or more)/ tensile proof stress)/ stress stress Hem No. in Mg Si Mg + Si ΣVi/V_(Al) × ΣVi × strength stress (tensile after BH increase worka- Classification No. Table 4 mass % mass % mass % 100 100 MPa MPa strength)] MPa MPa bility Inv. Ex. 51 27 0.33 1.08 1.41 0.63 33 202 91 0.450 207 116 2 Inv. Ex. 52 28 0.39 0.88 1.27 0.74 26 223 104 0.467 222 118 2 Inv. Ex. 53 29 0.52 0.70 1.22 0.78 21 223 103 0.462 207 104 1 Inv. Ex. 54 30 0.43 0.71 1.14 0.54 20 204 86 0.422 200 114 2 Inv. Ex. 55 31 0.61 1.02 1.64 0.88 37 224 107 0.477 221 114 1 Inv. Ex. 56 32 0.68 0.62 1.30 0.88 21 216 106 0.491 208 102 2 Inv. Ex. 57 33 0.44 1.05 1.49 0.78 34 224 104 0.464 219 115 2 Inv. Ex. 58 34 0.52 0.78 1.30 0.32 24 201 84 0.417 195 111 1 Com. Ex. 59 35 0.75 0.40 1.15 0.24 13 178 74 0.416 121 47 1 Com. Ex. 60 36 0.38 1.79 2.17 1.52 66 252 125 0.496 207 82 4 Com. Ex. 61 37 0.56 0.88 1.44 1.77 30 259 133 0.514 255 122 3 Com. Ex. 62 38 cracking occurred during hot rolling Com. Ex. 63 39 0.35 0.69 1.04 0.91 17 219 106 0.483 209 103 4 Com. Ex. 64 40 0.51 0.87 1.38 1.15 28 237 117 0.494 211 94 4 Com. Ex. 65 41 0.49 0.69 1.17 0.79 19 225 103 0.458 209 106 4 Com. Ex. 66 42 0.46 0.90 1.37 1.05 28 232 112 0.483 205 93 4 Com. Ex. 67 43 0.46 0.80 1.26 0.93 24 230 108 0.469 212 104 4

While the present invention has been described in detail with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present invention.

This application is based on a Japanese patent application filed on Mar. 31, 2014 (Application No. 2014-074045) and a Japanese patent application filed on Mar. 31, 2014 (Application No. 2014-074046), the entire contents thereof being incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide 6000-series aluminum alloy sheets which combine BH response and formability after room-temperature aging. As a result, the 6000-series aluminum alloy sheets are usable in applications extended to automotive panels, in particular, outer panels in which problems may arise concerning the design of beautiful curved-surface configurations, character lines, etc. 

1. An aluminum alloy sheet excellent in terms of formability and bake hardenability, which is an Al—Mg—Si alloy sheet comprising, in terms of mass %, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0% and Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable impurities, wherein a differential scanning calorimetry curve of the aluminum alloy sheet has an endothermic peak in a temperature range of 150 to 230° C., that is an endothermic peak corresponding to a dissolution of a Mg—Si cluster and that has a peak height of 8 μW/mg or less (including 0 μW/mg), and has an exothermic peak in a temperature range of 240 to 255° C., that is an exothermic peak corresponding to a formation of a Mg—Si cluster and that has a peak height of 20 μW/mg or larger.
 2. The aluminum alloy sheet excellent in terms of formability and bake hardenability according to claim 1, further comprising one kind or two or more kinds selected from the group consisting of Fe: more than 0% and 1.0% or less, Mn: more than 0% and 1.0% or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0% and 0.1% or less, Cu: more than 0% and 1.0% or less, Ag: more than 0% and 0.2% or less, and Zn: more than 0% and 1.0% or less.
 3. An aluminum alloy sheet excellent in terms of formability and bake hardenability, which is an Al—Mg—Si alloy sheet comprising, in terms of mass %, Mg: 0.3 to 1.0%, Si: 0.5 to 1.5% and Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable impurities, wherein a solid-solution amount of Mg+Si in a solution, separated by a residue extraction method with hot phenol is 1.0 mass % or more and 2.0 mass % or less, and wherein atom aggregates observed with a three-dimensional atom probe field ion microscope satisfy conditions that either or both of an Mg atom and an Si atom are contained therein by a total of 10 pieces or more and that, when any atom of the Mg atom and the Si atom contained therein is used as a reference, a distance between the atom as the reference and any atom among other atoms adjacent thereto is 0.75 nm or less, and regarding the atom aggregates, an average volume proportion (ΣVi/V_(Al))×100 is in a range of 0.3 to 1.5%, the average volume proportion (ΣVi/V_(Al)) being a proportion of the total volume of the atom aggregates, in terms of the total volume ΣVi obtained by summing up volumes of the individual atom aggregates Vi (=4/37πr_(G) ³) calculated from a Guinier radius r_(G) of the individual atom aggregates each regarded as a sphere, to a volume V_(Al) of the aluminum alloy sheet measured with the three-dimensional atom probe field ion microscope, wherein an average volume proportion (ΣVi_(1.5 or more)/ΣVi)×100 is 20 to 70%, the average volume proportion (ΣVi_(1.5 or more)/ΣVi) being a proportion of a total volume ΣVi_(1.5 or more) obtained by summing up volumes V_(1.5 or more) of atom aggregates each having the Guinier radius r_(G) of 1.5 nm or larger to a total volume of the atom aggregates ΣVi.
 4. The aluminum alloy sheet excellent in terms of formability and bake hardenability according to claim 3, further comprising one kind or two or more kinds selected from the group consisting of Fe: more than 0% and 1.0% or less, Mn: more than 0% and 0.4% or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0% and 0.1% or less, Cu: more than 0% and 0.4% or less, Ag: more than 0% and 0.2% or less, and Zn: more than 0% and 1.0% or less. 